Question: مَنْ كَتَبَ الدَّرْسَ؟ — Who wrote the lesson?
Answer: خَالِدٌ — Khālid did.
Understood: كَتَبَ خَالِدٌ الدَّرْسَ.
خَالِدٌ: nominative subject of the omitted past verb.
الدَّرْسَ: the object of the verb in the underlying question.
This additional exercise parallels the book’s “مَنْ ضَرَبَ؟ — زَيْدٌ”; the contextual question determines the missing wording.