The correct answer is C. The.
The word “the” is used to refer to a specific person, place, thing, or idea that has already been mentioned or is known to the listener or reader. In this case, the sun has already been mentioned in the sentence, so we use the definite article “the” to refer to it.
The word “an” is used to refer to a general person, place, thing, or idea. It is used before singular nouns that begin with a vowel sound. In this case, the sun is a singular noun that begins with a consonant sound, so we do not use the indefinite article “an”.
The word “a” is used to refer to a general person, place, thing, or idea. It is used before singular nouns that begin with a consonant sound. In this case, the sun is a singular noun that begins with a consonant sound, so we could use the indefinite article “a”. However, because the sun has already been mentioned in the sentence, we use the definite article “the” instead.
The word “that” is used to refer to a specific person, place, thing, or idea that has not been mentioned before. In this case, the sun has already been mentioned in the sentence, so we do not use the relative pronoun “that”.