Find the error in the last sentence of paragraph 5.

This is the traditional grammar rule that is assessed in many current tests: "The subject, student, is singular, so it requires a singular pronoun." Since the gender of the student is unknown, his or her would be appropriate.
Please note, however, that there is a trend among current publishers of dictionaries and writing style guides to accept the use of a singular "they." Our advice to you, as a test-taker, is to look over all answer choices and, if there is another applicable answer choice, evaluate which error would be the most severe, in light of this information. In this particular question, this is the only applicable error. Since it is still considered an error by some grammar authorities, you would be advised not to choose "no error" in this case. You may also want to verify which rule is used by a particular test by referring to other prep materials.