The erroneous assumption isn’t that the atomic structure is crystalline rather than amorphous. Instead, according to the author, people learn that glass has an amorphous atomic structure, and erroneously assume that this means that solid glass flows like liquid glass would, thus causing window glass to flow slowly downward.

This is the erroneous assumption that has led people to the incorrect belief that window glass flows noticeably downward over time. According to the author, people learn that glass has an amorphous atomic structure, and assume that this means that solid glass flows like liquid glass does, while in reality, glass below the transition temperature behaves as a solid, even though it has an amorphous structure.

We are looking for the incorrect assumption that the author believes is what caused people to accept the myth that glass flows downward over time. According to the author, this misunderstanding comes from assumptions about the effects of the structure of the atoms in glass, not from assumptions about medieval glass making methods, so (C) is not the erroneous assumption that caused people to accept the incorrect belief.

We are looking for the incorrect assumption that the author believes is what caused people to accept the myth that glass flows downward over time. According to the author, this misunderstanding comes from assumptions about the effects of the structure of the atoms in glass, not from assumptions about differences between medieval and modern glass. Additionally, nothing in the passage indicates that it’s wrong to think that the transition temperature of medieval glass is the same as that of modern glass; we don’t even know if the author thinks that this is an erroneous assumption.

This isn’t an erroneous assumption––the author agrees that liquid and solid glass are thermodynamically different.