Here's how I worked out the correct answer:
The correct statement should facilitate the appropriation of paintings painted by minor artists under the name of major artists. Now, let's see which all statements serve this purpose.
A. Technically gifted seventeenth-century Dutch landscape artists developed recognizable styles that were difficult to imitate. (This would rather be a counter-argument as this, if true, would make it difficult for the paintings to be passed on under others' names)
B. In the workshops of major seventeenth-century artists, assistants were employed to prepare the paints, brushes, and other materials that the major artists then used. (While this could enable the appropriation, it's difficult that this would be a reason for wrong assigning of names, as suggested by the correct answer)
C. In the eighteenth century, landscapes by minor seventeenth-century artists were often simply thrown away or else destroyed through improper storage. (This does not explain why paintings by minor artists were assigned under names of major artists)
D. Seventeenth-century art dealers paid minor artists extra money to leave their landscapes unsigned so that the dealers could add phony signatures and pass such works off as valuable paintings. (This easily would be the cause that resulted in the mismatch in number of paintings by minor and major artists.
E. More seventeenth-century Dutch landscapes were painted than have actually survived, and that is true of those executed by minor artists as well as of those executed by major artists. (This wouldn't explain why the numbers did not match earlier)