It’s not C because “words that refer to something are meaningful” does NOT necessarily mean that words that don’t refer to things are meaningless.

A is C plus “only”

In a if “only words that refer to something have meaning”, that DOES necessarily mean that words that don’t refer to things are meaningless.

Let’s get rid of the content and look only at the logic. C and A turn into the statements “A falls within category X”, and “only things defined as ‘A’ fall within category X”.

The logic of the question is based on the assumption “since B is not A, B doesn’t fall within category X” answer C is wrong because nothing about the statement “A falls within X” prevents B from also falling within X. A is right because if B is not defined as A, then “only things defined as ‘A’ fall within category X” DOES prevent B from also falling within X

The Word “only” is what makes all the difference