Let's think of an example, water boiling to form steam.
When the water boils, it obviously does not have the same shape, and liquids and gases don't have a definite shape, so shape is not the answer.
To force a phase change, water needs energy in the form of heat, so temperature is also not the answer.
The volume is also changed, and gases take up more space than liquids, so volume is also not the answer.
So, after ruling the other three out, the one we're left with is mass, and that is the answer: mass remains constant during a phase change
Don't hesitate to ask further questions!
Hope I helped!! xx
