Which one of the following sentences is punctuated correctly? a. della arrived on time, it was just two o'clock. b. she kept my surprise a secret, but she had a hard time doing it. c. darleen chose tara, holly, and, marlene as her lab partners. d. i found a large, smooth, rock on the beach yesterday.

The correct answer for the given question above would be option B. The sentence that is punctuated correctly is "She kept my surprise a secret, but she had a hard time doing it". The use of a comma is correctly used in this sentence, which is before the conjunction "but" which introduced a new clause.