Select the most appropriate option to fill in the blank and make a grammatically correct sentence.

One must keep _____ promise.

one's

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

The word "one" is a singular pronoun, and it can be used to refer to a person in general. In the context of the sentence, "one must keep one's promise," the word "one" is referring to a person in general. The possessive pronoun "one's" is used to show possession by the pronoun "one."

The other options are not as appropriate.

• Ones is the plural form of the pronoun "one." This is not the correct form of the pronoun to use in the sentence, because we are talking about one promise, not multiple promises.
• Its is the possessive pronoun for the third person singular neuter pronoun "it." This is not the correct form of the pronoun to use in the sentence, because we are not talking about a thing, we are talking about a person.
• The is the definite article. This is not the correct form of the pronoun to use in the sentence, because we are not talking about a specific promise, we are talking about any promise.