Which one of the following sentences is punctuated correctly?

AShe kept my surprise a secret, but she had a hard time doing it.
BDarleen chose Tara, Holly, and, Marlene as her lab partners.
CI found a large, smooth, rock on the beach yesterday.
DDella arrived on time, it was just two o'clock.

Verified Answer :

b darleen chose tara holly and marlene as her lab partners