The sentence's diction that best reflects a positive attitude toward the subject is "School uniforms benefit student learning in classrooms".
A sentence's diction is related to the word choice and the style that the writer of that sentence uses. Therefore, the term 'diction' is connected to the way in which someone writes. All the sentences provided share the same subject, which is "school uniforms"; however, they reflect different attitudes toward this subject. The one that presents a positive attitude is "School uniforms benefit student learning in classrooms" because it makes reference to an advantage of wearing a school uniform. Moreover, this has been clearly expressed by means of the word 'benefit'.