The constructions using being are awkward and wordy. Answer D is an example of a dangling modifier. Only Answer E is clear, and in the active voice throughout.

There are two main issues in this question.
(1) There are two actions that are being suggested and these should be parallel: (a) to decrease their debt (b) to increase their asset base.
(2) The use of the word should is redundant since the sentence (with the words require that) already carries the implication that banks need to take certain action.
A. the word should is redundant since the verb to require and the context of the sentence indicate that the suggested actions should be taken
B. the phrase it may be a good idea requiring that... is not correct as a verb (i.e., to require) is needed
C. the phrase the decreasing of debt and an increase is not parallel
D. the infinitive form of the verb to decrease cannot be used after requiring that all large and highly leveraged banks as it is awkward and not idiomatic
E. the phrase banks ... to decrease ... [to] increase is parallel since both verbs are used in the infinitive form (the second to is implied); the unnecessary word should is omitted

There are three main issues being tested in this question.

(1) The phrase their home is redundant as the word their could be eliminated
(2) The phrase other tribal groups historically experienced (with its verb in the simple past tense) must be replaced with a phrase that uses the past perfect tense (i.e., historically had experienced). The past perfect tense must be used since we are dealing with two actions in the past where one action came before another (i.e., the historic ability to find food predated the decision to move). The word historically makes it clear that the ability to find food referred to the period of time before the individuals decided to move in 2007.
(3) The word less cannot be used with difficulties since difficulties can be counted and the proper word for countable items is fewer
A. the word their is redundant; the word less should be replaced by the word fewer; the simple past tense (experienced) should be replaced by the past perfect tense (had experienced)
B. the simple past tense (experienced) should be replaced by the past perfect tense (had experienced)
C. the word their is redundant; the simple past tense (experienced) should be replaced by the past perfect tense (had experienced)
D. many indigenous people...left home not in search of other tribal groups (as this option incorrectly states), but for land where other tribal groups historically had experienced fewer difficulties finding food
E. the unnecessary word their is properly eliminated; the past perfect tense is correctly used; the word fewer is used with countable items

There are two major issues with the sentence as it was originally written:
(1) This sentence improperly uses much to describe a countable quantity (i.e., pollutants) when many should be used instead. In proper English, much is used for uncountable quantities (e.g., much of the water) while many is used for countable quantities (e.g., many apples, many gifts).
(2) The subject of the sentence (untraceable pollutants, which is plural) does not agree with the verb of the sentence (comes, which is singular).
A. Much is wrongly used to describe a countable quantity when many should be used instead; the subject (pollutants) does not agree with the verb (comes)
B. Much is wrongly used to describe a countable quantity when many should be used instead
C. the subject (pollutants) does not agree with the verb (comes)
D. Much is wrongly used to describe a countable quantity when many should be used instead; the phrase to kill fish and harming humans is not parallel (i.e., to kill is not parallel with harming, which should be harm)
E. Many is correctly used with a countable quantity; the subject and verb are both plural

There are two major issues with the sentence as it was originally written:
(1) This sentence improperly uses much to describe a countable quantity (i.e., pollutants) when many should be used instead. In proper English, much is used for uncountable quantities (e.g., much of the water) while many is used for countable quantities (e.g., many apples, many gifts).
(2) The subject of the sentence (untraceable pollutants, which is plural) does not agree with the verb of the sentence (comes, which is singular).
A. Much is wrongly used to describe a countable quantity when many should be used instead; the subject (pollutants) does not agree with the verb (comes)
B. Much is wrongly used to describe a countable quantity when many should be used instead
C. the subject (pollutants) does not agree with the verb (comes)
D. Much is wrongly used to describe a countable quantity when many should be used instead; the phrase to kill fish and harming humans is not parallel (i.e., to kill is not parallel with harming, which should be harm)
E. Many is correctly used with a countable quantity; the subject and verb are both plural