Question

if you place a crystal substance on a tabletop, return to view it after some time passes, and find it as a powered substance, which of the following processes most likely took effect?

Answer 1

The process which has taken place is called CRYSTALLIZATION.
Generally, crystallization is the process by which solid crystals are precipitated from solution. Crystallization can also occur when a crystal melt or when a crystal get deposited directly from a  gas, although these cases are rarer compare to crystals forming from solutions.