After better$-$than$-$expected quarterly sales, the CFO of Apps Incorporated e$-$mails Belden, a top salesman at the company, and promises to give stock options to Belden for his hard work during the previous year. Belden is grateful for the promise, and writes a response e$-$mail thanking the CFO for his generosity. This promise is

a$.$ enforceable because it is a new contract formed through offer and acceptance.

b$.$ unenforceable because it is an illusory promise.

c$.$ enforceable, but only under the doctrine of implied contract.

d$.$ unenforceable because it is based on past consideration.