The correct option is C grow up
A phrasal verb is a verb combined with an adverb or preposition to
provide a meaning different from the literal meaning of that individual
verb. The underlined phrase 'grow back' means 'to begin growing again after being cut or damaged' as in 'he trimmed my hair too short, but they will grow back soon'. This phrase is not relevant here as people cannot be cut and grown back, hence option A is wrong. 'Grow up' means 'to grow into an adult' and is relevant to the statement, as the speaker wants his cousin to become an adult, hence option C is correct. 'Grow into' also refers to a development, but requires a phrase to answer the question 'grow into what?', like 'I can't wait to see my little cousin grow into a fine young lady'. Since the phrase is not followed by any other words in the statement, option B is incorrect. 'Grow apart' means 'to become estranged', it isn't relevant here, hence option D is incorrect.