The correct option is
B The/the/a/the
OPTION "A" is the correct answer.
EXPLANATION:
There are two types of articles:
DEFINITE ARTICLE (THE) : Article 'the' is used to refer to a DEFINITE thing or person.
INDEFINITE ARTICLE(A/AN): Article 'a/an' is used to refer to a thing or person in general. For instance: man, door etc.
Article 'a' is used with consonant sounds whereas article 'an' is used with vowel sounds.
In the FIRST Blank, article "the" will be used because article "the" is used with the things only one (the sun, the moon).
In the second blank, article "the" will be used because 'rain' is definite here.
In the third blank, article "a" will be used as the sentence is referring to something general.
In the fourth blank, article "the" will be used because here the sky is specifically mentioned for a specific day.