The fire destroyed what was in the museum.
what引导宾语从句,在宾语从句中作主语,意为：所……的事/物
题意：大火把博物馆里所有的东西都烧尽了.
which引导宾语从句时,多作定语,其后一般要跟一个名词.如果其单独作主语,前面应有可选的范围.如：
There are so many good toys that I don't know which I should buy.(此时which指前文中所说的toys中的其中一个或几个）
根据题意,此题中不宜填which.