A and D each sound appealing, but I imagine the reason you are supposed to eliminate A is because it is just as possible/likely that non-taller students cheated, so my answer is D. The other answer choices seem less appealing.
1.Joseph gives a math test to all the students in a middle school and notices that taller students have higher scores. He concludes that taller people are better at math. This conclusion is
A) not justified, since the taller students could have cheated.
B) justified, especially if it can be repeated on a second test.
C) not justified, since taller students have had more math in school.
D) justified, assuming that the test was given fairly to all students.
Fortunately, one does not have to solve such problems as a part of mathematical practice. The question is not formulated nearly precisely enough to qualify as a valid mathematical problem. The first thing that came to my mind regarding "He concludes that taller people are better at math" was, "What is the set of taller people Joseph considered?". If it is all people, including non-students, then this conclusion is definitely unjustified, but not for the reasons given in the question.
On the other hand, making similar questions precise is often a part of mathematical practice.