A current psychological theory seeks to explain how children become

if you break down the passage into its parallelism structures, then what we get is as follows:

A current psychological theory seeks to explain how ......., when ......, and ______ ....

Here, we have relative clauses that are in parallel with one another. Hence, what follows the conjunction 'and' must be a relative clause.

Since (C), (D), and (E) are not relative clauses, we can eliminate (C), (D), and (E).

What we are now left with are (A) and (B).

(A) what personality type is most prone to rely - (A) fits perfectly well into the parallelism structure as shown above. Moreover, (A) has no error in it. Hence, (A) is the right answer choice.

(B) what is the personality type most prone to relying - If you attach (B) to the main stem of the passage, you get to realize that (B) is unnecessarily wordy, in reference to (A). Moreover, the present continuous form of the verb 'rely' seems to indicate that the action of this verb is happening at the same time as the main verb of the sentence ('explain'). The usage of the -ing form of the verb does not make sense. Hence, we can eliminate (B).

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