Which sentence is an example of a pun?

a. i want to look for my watch, but i can never seem to find the time.
b. by making cupcakes for them, i can kill two birds with one stone.
c. it is often wise to keep your friends close and your enemies closer.
d. she explained the novel so well that she hit the nail on the head.
she did not with dinner because too many cooks can spoil the broth.