The correct answer is: C. a
The word “hour” is a singular noun, so it takes the indefinite article “a”.
The indefinite article “a” is used before singular nouns to refer to a general or unspecified person, place, thing, or idea.
In this case, the speaker is not referring to a specific hour, but rather to any hour in the future when the person will arrive.
The other options are incorrect because they are definite articles. The definite article “the” is used before singular nouns to refer to a specific person, place, thing, or idea that has already been mentioned or is known to the listener.
In this case, the speaker has not mentioned a specific hour, so the definite article “the” would not be appropriate.
The article “an” is used before singular nouns that begin with a vowel sound. The word “hour” does not begin with a vowel sound, so the article “an” would not be appropriate.
The article “two” is a number, not an article. It would not be used in this sentence.