Assertion (A) is valid because the statement "The details of the incident" contains a grammatical error related to subject-verb agreement. Since "Details" is plural, the verb should be "were", not "was".
Reason (R) is also accurate. In phrases of the form "Noun + Preposition + Noun," the verb must agree with the initial noun (here, "details"), which is plural. Consequently, Reason (R) accurately explains the grammatical error.