The given question involves understanding subject-verb agreement and its application in the provided sentence. Let's analyze the assertion and reason:
The assertion states that the sentence "The details of the incident was not known." does not adhere to subject-verb agreement rules and is therefore grammatically incorrect.
Therefore, Assertion (A) is true.
The reason given is: "In 'Noun + Preposition + Noun' constructions, the verb agrees with the number and person of the noun preceding the preposition."
Therefore, Reason (R) is also true.
Both Assertion (A) and Reason (R) are true, and Reason (R) indeed explains why the assertion claims the sentence is incorrect. Therefore, the correct answer is:
Choose the correct option to fill in the blank: She is good ………….. mathematics.