I believe that the word is more often used as a non-countable noun. I'm with you and your son. That being said, it seems to be an odd choice to put on a test.
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NikooHi. I think 'candy' can be used as a countable noun when you want to refer to pieces of candy. I guess that's what your son's teacher had in mind.But that isn't what the teacher put on the test. The teacher was careless and should have realized the mistake if it was brought to his or her attention.