The correct answer is C. Change they should have a positive attitude to positive in their attitude.
Explanation:
In the sentence "Athletes benefit from regular practice, a healthful diet, and they should have a positive attitude" there is faulty parallelism because the author uses the same structure in "regular practice, a healthful diet" (adjective+ noun) but not in the last element "they should have a positive attitude" (noun+ modal verb+ verb+ adjective+ noun). This means there is no one grammatical pattern, and therefore there is no parallelism. Moreover, this can be corrected by omitting the section "they should have" and using only "a positive attitude" because in this case, all the elements listed would contain an adjective and a noun, which means there is parallelism.
