The correct option is A a
George works three mornings a week. It hasn't been specified which week, it's a general week, or every week. Being a non-specific, indefinite noun the article 'a' is the correct option, 'an' is incorrect because the noun does not begin with a vowel. 'The' is incorrect because it isn't specified which week. Option D is incorrect because an indefinite article is required before an indefinite noun.