The correct option is C the
When the noun 'teacher' is followed by the clause 'we have to impress' that identifies it, the noun is made specific and we use the definite article 'the' 'The' is the definite article which we use when the noun is known, and when we want to lay emphasis on the noun. Hence option C is correct. Options A and B are indefinite articles which are used for nouns that are not specific, or not known to the reader or listener since the noun here is specific, these options are incorrect. Option D is a pronoun which is used for plural nouns, whereas what we need here is the article 'the' before the noun. Pronouns are used to replace nouns that have been mentioned before, we cannot place a pronoun before the noun, it is grammatically incorrect. Hence option D is incorrect.