Final answer:
To convert the passive sentence 'The cup was broken' to active voice, you identify who broke the cup and rephrase it as 'Someone broke the cup.'
Step-by-step explanation:
To change the sentence "The cup was broken." from passive to active voice, you need to identify the agent performing the action, and then rephrase the sentence to place the agent before the verb. If we assume that 'someone' broke the cup, the sentence in active voice would be: "Someone broke the cup."