Question

his command was promptly obeyed.convert into active voice.

Answer 1

The given sentence is in passive form. Here in this sentence "object" is not given. Here u can choose any object as "you" is object.

We have to change object into subject and vice-versa. The sentence is in past form so active sentence also will be in simple past.

ANSWER

You promptly obeyed his command.

Answer 2

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They obeyed his command promptly ;)