The correct answer is C. Mom said to put in three-fourths cup of milk. The easiest way to explain when to hyphenate a fraction and when not to is this: when a fraction is followed by the word OF, you don't have to place a hyphen there. But if it is followed by a noun (like in sentence C, where the fraction three-fourths is followed by a noun CUP), then you have to place a hyphen there. Hope this helps!
