Which of the following sentences demonstrates correct subject-verb agreement?
A. The children goes to bed later durning the summer.
B. Fiona and Robert is riding the bus to the park this afternoon.
C. They washes the car on sunny weekend days.
D. Victor plays the piano almost every afternoon

D. Victor plays the piano almost every afternoon

Explanation:

Subject-verb agreement occurs when a singular subject is followed by a singular verb, or when a plural subject is followed by a plural form verb. In that respect, the sentence contains a singular subject, Victor, and a singular verb, plays.

The rest of the sentences are incorrect because they contain subjects and verbs which do not match in number. The correct version is the following:

A. The children go to bed later durning the summer.

B. Fiona and Robert are riding the bus to the park this afternoon.

C. They wash the car on sunny weekend days.

The correct answer is D. "Victor plays the piano almost every afternoon"

Explanation:

As in many languages, in English, the subject has to agree or concord with the verb; this is called subject/verb agreement. That is, if the subject is singular, the verb MUST be singular, and if the subject is composed of two or more people/objects, the verb MUST be plural. If not, there is a serious grammar mistake, which makes ungrammatical the complete sentence.