Option(A) is correct "Whether" is correct because the question concerns a choice not a condition. With the expression "the number of" a singular verb is needed and hence "was" is correct. "Liable" is used in expressions such as "liable to prosecution" and not for expressions of possibility.

The verb claims should be followed by one of two patterns: (1) that followed by a subordinate clause (2) claims followed by an infinitive (set apart by commas).
A. claims is correctly followed by that and a subordinate clause set apart by commas
B. the phrase claims of the ability, ..., of returning is neither grammatical nor idiomatic
C. the phrase claims of being able to return, ..., the investment is neither grammatical nor idiomatic
D. the phrase to be...able to return should not be split up as it creates an awkward construction; to be able to return can be made concise by removing to be able
E. the phrase returning the investment bank back is redundant as the words returningand back convey the same idea; the phrase he is capable of can be shortened to the phrase he can

There are two main issues being tested in this sentence.
(1) When using like or unlike, you must compare like parts (e.g., compare arguments with arguments). The original sentence improperly compares the team of lawyers with the government's argument.
(2) The expression, which modifies the term that is immediately before it. For example, the phrase the argument from the petitioner, which rested on a questionable interpretation of a bill that only recently passed Congress is incorrect since it was not the petitioner that rested on a questionable interpretation, but rather the argument that rested on a questionable interpretation.
A. the sentence illogically and improperly compares unlike parts (i.e., it compares the team of lawyers with the government's argument)
B. the phrase the argument from the petitioner, which rested on a questionable interpretation of a bill that only recently passed Congress is incorrect since it was not the petitioner that rested on a questionable interpretation, but rather the argument that rested on a questionable interpretation
C. the sentence properly compares like parts (i.e., it compares the petitioner's argument with the government's argument); , which rested on... properly and logically modifies the phrase it follows
D. the phrase whose case rested on is illogical since whose (which should modify a person) is actually modifying an argument
E. the original sentence, which reads a bill that only recently passed Congress, is perniciously changed to a new sentence, which reads a bill that recently only passed Congress; the difference in meaning between a bill that recently only passed Congress (meaning it did not become law) and a bill that only recently passed Congress (meaning it passed Congress a short time ago) is significant

The superlative form most scary must be used instead of the comparative form more scary since the author is indicating that among all the threats, a specific threat is the most scary.
A.more must be replaced by most as the superlative form (most) must be used since the author is not comparing threats but stating that a single threat is the scariest
B.The superlative form is correctly used
C.Although the superlative form is correctly used, this choice makes the sentence unduly long; the phrase possibly, perhaps is awkward and not idiomatically correct
D.While a superlative is used, the sentence is exceedingly awkward as the phrasepossibly the greatest scary is not idiomatic
E.The superlative form is not used; the phrase possibly the great scary as this scenariois awkward and not idiomatic

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.