Here's the official explanation provided by the GMAC for this question:
The elliptical construction when forced to sell them in order to raise money should be immediately followed by a noun, noun phrase, or pronoun to make it clear who or what was forced to sell the portraits. The original version is structured in a way that clearly indicates who was forced to sell the portraits and who made the copies.
Option A: Correct. Using appropriate verb forms and sentence structure, this clearly communicates that he (Rembrandt) made the copies, and the position of the pronoun directly after the modifying phrase forced to sell them in order to raise money makes it clear that Rembrandt was the one who was forced to sell them.
Option B: The position of the adverb first causes it to intervene awkwardly between the phrase forced to sell them … and the pronoun he. Furthermore, the connotations of first he made are somewhat different from those of the phrase he first made. The latter more clearly indicates that he made the copies before selling the portraits, whereas first he made tends to suggest that making the copies was the first of a series of things that he did.
Option C: Forced to sell them illogically modifies more than twenty. Saying merely that they were copied, without telling who copied them or caused them to be copied, makes the connection between the copying and Rembrandt’s treasuring the portraits unclear.
Option D: Forced to sell them illogically modifies copies of more than twenty. Saying merely that the copies were made, without telling who made them or caused them to be made, makes the connection between the copying and Rembrandt’s treasuring the portraits unclear.
Option E: The present tense copies is inconsistent with the overall past perspective of the sentence.
The correct answer is A.
Please note that I'm not the author of this explanation. I'm just posting it here since I believe it can help the community.