Rewrite the sentence in the active voice.
The telephone had been invented by Graham Bell.

Correct option is C

The correct option is (c) Graham Bell had invented the telephone.
Definition of Active Voice: In active voice, the subject of the sentence performs the action.
Definition of Passive Voice: In passive voice, the subject of the sentence receives the action.
Tense identification: The sentence is in Past Perfect Tense.
Rule: In Past Perfect Tense, the structure for changing passive to active is:
Passive voice: Object + had + been + V3 + by + Subject.
Active voice: Subject + had + V3 + Object.
Corrected sentence: Graham Bell had invented the telephone.