Explanation:
The correct answer is option 'B' because the verb "plays" agrees with the subject "ninja" in number and person.

Subject-Verb Agreement:
In English grammar, subject-verb agreement refers to the correspondence between a subject and its verb in terms of number (singular or plural) and person (first, second, or third).

Noun:
In the given sentence, the noun is "ninja." The noun "ninja" is a singular noun and refers to a single person.

Verb:
The verb in the sentence is "plays." The verb "plays" is the third-person singular form of the verb "play."

Subject-Verb Agreement:
In subject-verb agreement, a singular subject requires a singular verb form. Since the word "ninja" is a singular noun, we need to use the third-person singular form of the verb, which is "plays."

Verb Forms:
The verb "play" has the following forms:
- Base form: play
- Present tense (third-person singular): plays
- Present participle: playing
- Past tense: played
- Past participle: played

Options:
Let's analyze each option to determine the correct one:
a) play: This is the base form of the verb and does not agree with the singular subject "ninja."
b) plays: This is the correct answer. It is the third-person singular form of the verb "play" and agrees with the singular subject "ninja."
c) playing: This is the present participle form of the verb and does not agree with the singular subject "ninja."
d) played: This is the past tense form of the verb and does not agree with the present tense meaning of the sentence.

Conclusion:
Based on subject-verb agreement, the correct answer is option 'B' because the verb "plays" agrees with the singular subject "ninja."