【正确答案】 D

【答案解析】Grammatical construction; Idiom
This sentence links two independent clauses; in such sentences, the clauses must normally be set off from each other (by a semicolon, for example), or else the second clause must be introduced by and or some other conjunction, not merely an adverb like thus. Also, a noun like domain normally is followed by the preposition of immediately preceding the noun phrase describing the domain.
A The second clause incorrectly lacks an introducing conjunction.
B The phrasing is best to be studied is awkwardly unusual and unidiomatic; is best studied is a better choice.
C The second clause incorrectly lacks an introducing conjunction. The phrase a "theory of mind" would refer to one particular theory rather than (as intended) to a theoretical domain.
D Correct. And introduces the second clause, which uses the concise wording best studied.
E The beginning of the second clause is redundant and wordy: the word and is unnecessary because the conjunction so is used, and best to be studied is unidiomatic.
The correct answer is D.