Choice "D" is correct. To constitute consideration, there must be a bargained for exchange of something of value. A detriment to the promisee or a benefit to the promisor constitutes value. A promise to pay a minor $500 to paint a garage constitutes a detriment to the promisee; the promisee is not otherwise bound to pay the minor $500 to paint the garage, and the minor's painting the garage constitutes valid consideration to support the promise to pay $500.
Choice "c" is incorrect. To constitute consideration, there must be a bargained for exchange of value. A detriment to the promisee or a benefit to the promisor constitutes value. Past acts generally do not constitute valid consideration because the acts were not bargained for. Thus, a promise to pay $1,000 in return for a prior performed act (not charging for services) is not supported by consideration.
Choice "a" is incorrect. To constitute consideration, there must be a bargained for exchange of value. A detriment to the promisee or a benefit to the promisor constitutes value. Here, there is no bargained for exchange. The promise is gratuitous and not supported by consideration.
Choice "b" is incorrect. To constitute consideration, there must be a bargained for exchange of value. A detriment to the promisee or a benefit to the promisor constitutes value. A promise to perform a pre-existing duty does not constitute valid consideration. Because a police officer already owes a crime victim a duty to catch the perpetrator, the police officer's promise to perform his duty does not constitute valid consideration.