Change into passive: ‘One should keep one’s promises’

C. Promise should be kept.

Here’s how the active to passive voice rule applies:

Active Voice: Subject (One) + Verb (should keep) + Object (one’s promises) (One is implied, not a direct object)

Passive Voice: Object (Promises) + Verb (to be + past participle) + By + Agent (Subject) (One is general and not a specific agent, so often omitted)

Explanation:

• Object: “One’s promises” becomes the subject in the passive sentence – “Promises”
• Verb: “Should keep” is a modal verb indicating obligation. In the passive voice, we can’t directly convert it. “Should” suggests recommendation, so we can use “be kept” (past participle of “keep”) to indicate the action on the promises.
• By + Agent: Since “one” is general and doesn’t specify who should keep promises, it’s often omitted in the passive voice.