Which of the following is an example of a unilateral contract?

Select one:

a$.$ Sam orders a meal in a restaurant.

b$.$ Joe returns a lost dog and claims the $$50$ reward.

c$.$ Joe pays $$50$ for groceries at a grocery store.

d$.$ Joe agrees to paint Sam's house for $$500.$

e$.$ Joe promises to give Harry his old boat when he gets his new one.