Volume is an unreliable measurement because

A. food expands or contracts in different temperatures.

B. gas weighs less than liquid.

C. there's no standard size of measuring cup or spoon.

D. solids can't be measured accurately by volume.

The answer would be: A. food expands or contracts in different temperatures.

Volume is directly related to temperature and inversely related to pressure. Different temperature / pressure will result in a different volume.

The law of conservation of mass state that mass can't be created or destroyed. That means the weight of something won't change. The same amount of molecule in gas and liquid should weight same.