The correct option is
A an/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 "a" will be used because 'apple' is a common noun.
In the second blank, article "a" will be used because 'day' is a common noun.
In the third blank, article "the" will be used because the speaker is mentioning someone specific/definite like a doctor who can be hypothetically kept away by eating an apple everyday.