The phrase "what is the best vitamin for hair growth" is a question seeking information about the most effective vitamin in promoting hair growth. It can be categorized as a noun phrase, as it functions as a subject or object in a sentence.
Biotin, a B vitamin, is often touted as the "best vitamin for hair growth" due to its role in keratin production. Keratin is a protein that forms the primary structural component of hair, nails, and skin. Biotin deficiency has been linked to hair loss and brittle nails.