Question

which would you refreeze? ice cubes that are starting to melt water that you want to make into ice cubes

Explanation:

The term "refreeze" implies cooling something that was previously frozen but has started to change state back to its frozen form. Ice - cubes that are starting to melt are the ones that were previously frozen and are now in the process of changing to a liquid state, so they would be the ones to refreeze. Water that you want to make into ice - cubes has not been frozen before in this context and thus is not being "refrozen".

Answer:

ice cubes that are starting to melt