Which example describes a one-tailed hypothesis?

A one-tailed hypothesis predicts the direction of the relationship or the difference between groups. In this case, the statement "Students score lower than the national average" specifies not just that there is a difference in scores but indicates the specific direction of that difference – that students will score at a level that is less than the national average.

This contrasts with a two-tailed hypothesis, which would simply postulate that there is a difference without specifying whether it is greater or lower. For instance, if the statement was simply about there being a difference in scores, it would not suffice to indicate a one-tailed hypothesis. Therefore, the fact that option B asserts a particular direction (lower scores) is what categorizes it as a one-tailed hypothesis.