Which of the following sentences demonstrates proper subject-verb agreement?
A. Measles usually spread through respiratory fluids.
B. The study of various sciences are compelling.
C. News travel fast.
D. Physics is a difficult subject to learn.
The sentence that demonstrates proper subject-verb agreement is D. "Physics is a difficult subject to learn." In this sentence, the singular subject "Physics" agrees with the singular verb "is."
Therefore, the Correct Answer is D.