The answer is the third option: Vous la mangez.
EXPLANATION: The first choice is grammataically incorrect since the "la" is misplaced and if you already know some French, it doesn't read well or sound right. The second choice uses a masculine "le" when the noun in the sentence (la bonne tarte- the good pie) is FEMININE. Finally, the last choice is also wrong because it uses a plural "les" when the noun in the sentence is singular.
Thus, only the third choice is correct.
