The answer is the butter contains a new arrangement of atoms.
When melting, the atoms breaks bonds between them, they spread further apart and therefore became liquid.
For A, butter does not change into another substance, nothing is added or combined.
For C, butter can turn into a solid again if we cool it down below it's melting point.
For D, butter still contains the same number of atoms, again, only they became further apart.
Therefore the answer is B.