Assertion (A) is correct because the sentence "The details of the incident" was not known” is grammatically incorrect due to subject-verb agreement. "Details" is plural, so the verb should be "were" instead of "was".
Reason (R) is also true. In constructions like "Noun + Preposition + Noun,” the verb agrees with the noun before the preposition (here, "details"), which is plural. Therefore, Reason (R) correctly explains why the sentence is grammatically incorrect.
Poetic Devices: Identify the poetic device used in the following line: "The wind whispered through the trees."