The trouble with this is that a scallop shell is only a couple of inches across.
The scallop is actually a type of clam, and it may be more accurate to describe Venus's shell as that of a giant clam – which can grow to as much as four feet across.
The only trouble with this is that giant clams only live in the Pacific and Indian oceans, and weren't known to Europeans until the early 16th century. (According to Wikipedia, they were documented in 1521 by the Venetian scholar Antonio Pigafetta, who travelled with Magellan.) The Birth of Venus was painted in around 1486.
All of this is of course academic, and we should allow some artistic licence. But when it comes to quiz questions, "clam shell" is at least as accurate as "scallop shell", and should certainly not be disallowed as an answer. In fact, the safest thing is to allow any sort of shell – as the setter did on this occasion.
© Macclesfield Quiz League 2019