His parents did not pay . . . . . . . attention to his studies.

The correct answer is D. proper.

The other options are not correct because:

• A. suitable: This means “fitting or appropriate.” It does not make sense to say that his parents did not pay fitting or appropriate attention to his studies.
• B. just: This means “fair or impartial.” It does not make sense to say that his parents did not pay fair or impartial attention to his studies.
• C. remarkable: This means “very unusual or outstanding.” It does not make sense to say that his parents did not pay very unusual or outstanding attention to his studies.

The word “proper” means “correct or appropriate.” It is the most accurate word to describe the parents’ lack of attention to their son’s studies.