This question tests the ability to identify the فاعل (subject/doer) in an Arabic verbal sentence.
In the sentence شرح المعلمُ الدرس (The teacher explained the lesson), the grammatical analysis is:
شرح is the فعل ماضٍ (past tense verb), meaning "explained." The verb is not the doer of the action; it describes the action itself.
المعلمُ is the فاعل (subject), the one who performed the action. It carries the ضمة (nominative marker), which is the required case for the فاعل.
الدرس is the مفعول به (direct object), meaning "the lesson" - the thing that was explained.
The فاعل always comes after the verb in a verbal sentence (جملة فعلية) and is always in the nominative case (مرفوع). To identify it, ask: who performed the action? The teacher is the one who explained, making المعلم the فاعل.
