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

Correct option is C

Explanation: The passive sentence "The telephone had been invented by Graham Bell" indicates a completed action in the past before another past event, best conveyed in active voice as "Graham Bell had invented the telephone." This simplifies the expression while keeping the past perfect essence where necessary.
Conversion Rule: Past perfect
Subject + had + V3 + object -> <- Object + auxiliary verb (had) + been + past participle + by + subject.
Example: The theory had been developed by Einstein. -> Einstein had developed the theory.