The identity of the nouns has not been mentioned, hence both of them are non-specific and singular, as they have been mentioned separately, hence an indefinite article is used 'a' and 'an' are indefinite articles where 'a' modifies a noun beginning with a consonant and 'an' modifies a noun beginning with a vowel. When a noun is modified by an adjective, then the initial of the adjective determines which indefinite article is to be used.
Option D is correct because the article 'a' modifies the nouns 'content writer' and 'graphic designer' that begin with the consonants 'c' and 'g', respectively. Option A is incorrect because the noun "graphic designer" does not begin with a vowel.
Option B is incorrect because the noun "graphic designer" is not specific, hence we wouldn't use 'the' before it.
Option C is incorrect because the noun 'writer' is not specific, and the noun "graphic designer" does not begin with a vowel.