Option A - 'A' can't be used for uncountable nouns. Thus option A is incorrect.
Option B - 'An' can't be used for nouns that have consonant sound. Thus option B is incorrect.
Option D - 'The' is used for definite nouns. As we do not know which kind of butter the patient must not eat, we can't use 'the' here. Thus option D is incorrect.
Option C - As 'butter' is a general noun, which is uncountable, we do not need an article to describe it. Thus option C is the correct answer.
'A heart patient must not eat butter.'