Margaret Mows the Lawn for Her Parents

Margaret mows the lawn for her parents.After she has finished mowing the lawn,Margaret's father tells her he will pay her $50.This contract is

A) unenforceable because Margaret has already mowed the law and past consideration is no consideration.
B) enforceable because Margaret's father has a moral obligation to keep his promise to her.
C) unenforceable because minors may not enter into contracts.
D) unenforceable due to the preexisting duty rule.