Select the option that expresses the given sentence in passive voice.

One should keep one's promise.

Promises should be kept.

The correct passive voice form of the given sentence "One should keep one's promise" is Promises should be kept.

The active voice of the sentence is "One should keep one's promise." This sentence is in the simple present tense, which is used to describe an action that happens regularly or happens now. The passive voice of the sentence is formed by putting the subject of the active sentence after the verb "to be" and the past participle of the main verb. In this case, the subject of the active sentence is "promise" and the main verb is "keep." The past participle of "keep" is "kept." So, the passive form of the sentence is "Promises should be kept."

The other options are incorrect because:

• One's promise should be kept by one." This sentence is in the active voice, but it is in the imperative mood. The imperative mood is used to give a command or make a request. However, the sentence is not a command or a request.
• Promises should be kept by one." This sentence is also in the active voice, but it is in the present tense. The active voice of the sentence should be in the passive voice, since the subject of the sentence is not the one who is doing the action.
• One should keep their promise." This sentence is grammatically correct, but it is not the correct passive voice form of the sentence. The word "their" is used to refer to a group of people, but the subject of the sentence is "one," which is singular.