Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R).
"The details of the incident was not known."

Assertion (A) :
The above sentence does not adhere to the requirement of subject-verb agreement and hence it is a grammatically incorrect sentence.

Reason (R) :
In "Noun + Preposition + Noun" constructions, the verb agrees with the number and person of the noun preceeding the preposition.
In the light of the above statements, choose the correct answer from the options given below:

Assertion (A): The sentence "The details of the incident was not known" does not adhere to the requirement of subject-verb agreement and is grammatically incorrect.
This is correct because the subject "details" is plural, so the verb should be "were" instead of "was."

Reason (R): In "Noun + Preposition + Noun" constructions, the verb agrees with the number and person of the noun preceding the preposition.
This is also correct because in the given sentence, the noun preceding the preposition "of" is "details," which is plural. Therefore, the verb should match the plural subject.

Conclusion: The Reason (R) correctly explains the grammatical error identified in Assertion (A). Thus, the correct answer is:
Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of Assertion (A).