First lets figure out how many questions each person answers.
Amna answered 1/3 of them, so we divide 84 by 3: She answered 28 questions.
Buthaine answered 1/4 of the questions, so we divide 84 by 4: He answered 21 questions.
Badria answered the rest, so we add 28 + 21 and get: 49. Now we subtract 49 from 84 and get: 35
Badria answered the most questions, and answered a total of 35. The answer is D.
Hope that helps!