To cut the chase:
- C is just wrong, un necessary "a"
- B, D, E are grammatically correct but B and E are awkward: past participle is generally less preferred. “Thus X is best studied as…” > “Thus X is to be studied as…”
- Between A and D, A is not correct as we need conjunction to separate two clauses
- Answer = D
The key to solve this question is to understand:
- The sentence structure here can be dissected as: The type of behaviour … comes within the domain … thus is best studied as
- “Thus” here acts as conjunctive adverb (herefore, however, nevertheless, consequently, for example, on the other hand, moreover, besides, accordingly)
- The rule is we need to use “and”, or a semicolon in front of conjunctive adverb to separate two independent clauses – I think this is the crux of the questionSomeone please correct me if I’m wrong.