In “مَنْ ضَرَبَ؟ — زَيْدٌ,” an active verb is understood but omitted from the spoken reply: “ضَرَبَ زَيْدٌ.”
In “ضُرِبَ زَيْدٌ,” the verb is explicitly present in the passive form, and Zayd is the passive subject.
The distinction is crucial: the former involves ellipsis, while the latter involves passive voice.