Fill in the blanks with the most appropriate option:-

One should keep_______ promise.

ones

The most appropriate option to fill in the blanks is one's.

The word "one's" is a possessive pronoun that refers to something that belongs to one person. In the context of the sentence, "One should keep one's promise", the promise belongs to one person. The other options are not as appropriate.

• Ones is a plural form of the word "one". This does not make sense in the context of the sentence, as the promise does not belong to multiple people.
• His is a possessive pronoun that refers to something that belongs to a male person. This does not make sense in the context of the sentence, as the promise does not belong to a male person.
• Her is a possessive pronoun that refers to something that belongs to a female person. This does not make sense in the context of the sentence, as the promise does not belong to a female person.
• Once is an adverb that means one time. This does not make sense in the context of the sentence, as the promise is not something that is done one time.