。
She looks very young.
Literal
She [topic-は] very young appears.
若く is the adverbial form of the i-adjective 若い ('young'), formed by swapping the final い for く. Pairing it with 見える gives the i-adjective version of 'look ADJ' — Japanese conjugates the linking part of speech to match the adjective class (く for i, に for na). Looking younger than one's age (若く見える) is a culturally weighted compliment in Japan, where age-conscious self-care is a common topic.