Since Jennifer was a year younger than her friends, she had a hard time keeping . . . . . . . . with them. A. up B. of C. at D. None of these

The other options are not correct because they do not make sense in the context of the sentence. The word “of” means belonging to or connected with. The word “at” means in or to a particular place. The word “none of these” is not a correct answer because it does not specify which option is correct.