Bunuel
Idioglossia is a phenomenon, incompletely understood at best, where two persons develop a unique and private language with highly original vocabulary and syntax.
(A) where two persons develop a unique and private language with
(B) when two persons develop a unique and private language having
(C) in which two persons have unique and private language development with
(D) having two persons who develop a unique and private language that has
(E) in which two persons develop a unique and private language with
We can eliminate answer choices by looking at the splits first:
Eliminate (A): Incorrect use of 'where'; 'where' modifies a location.
Eliminate (B): Incorrect use of 'when'; 'when' modifies time.
Eliminate (D): 'Having' is a present participle that should modify a
noun phrase; "Idioglossia is.." is a
noun clause.
Eliminate (C): Meaning is distorted here; two persons have: (i) unique and (ii) private language development do not explain the phenomeon as originally intended.
The answer is choice (E): Two persons
develop a: (i) unique and (ii) private language, which explains the phenomenon perfectly.