prasannajeet
Some people may believe that a person, who does not behave normally in public, should be declared insane; however its legal definition is someone who is unable to distinguish right from wrong.
(A) its legal definition is someone who is unable- Pronoun ref not correct
(B) legally, it refers to a person who is unable- Pronoun ref not correct
(C) it is legally defined as the inability-wordy
(D) the legal definition of insanity is the inability-Correct
(E) legally, they refer to someone who is unable - Pronoun must be singular
Please correct me if I am going wrong somewhere........................
Yes prasannajeet, you are correct. In GMAT if you see an underlined part starting with a pronoun, it is highly probable that the sentence is testing pronoun issues.
As far as the explanation is concerned
(A) contains a meaning error "its legal definition is someone" the definition can not be someone.
(E) Clearly contains a pronoun number error.
Choices (B), (C) and (D) are not exactly grammatically incorrect, however (D) conveys the meaning without any ambiguity and hence the correct answer choice.
Both (B) and (C) have the ambiguous pronoun "its".
Vercules