Question

change in passive voice-

One should keep one's promise.

Answer 1

Answer:-

This sentence means that every body should keep his/ her promise. It doesn't mean about one or some persons.

So, the answer is ( Promises should be kept ) but ( by one) should not be used.

[AV: One should keep one's promise.]

[PV: Promise should be kept.]

Answer 2

One's promises should be kept .

Details :

An active voice has its object after the subject .

A passive voice has its subject after object .

This was changed and the subject is kept at the end so as to change the voice.

The meaning of the sentence is unaltered.