Tricky one this!
beautify: add beauty to.
he (who) beautifies
1-It's you who beautify the clothes, not the clothes which beautify (they beautify) you!
*2-It's you who beautifies the clothes, not the clothes which beautify you! Not good: you beautify, not 'Who beautifies?' nor '*you beautifies' The relative pronoun 'who' represents 'you', second person singular or plural. Cf Who is this damsel, who beautifies my house so?
3-It's you who beautify the clothes, not the clothes that beautify you!
If you put on a pirate accent, I suppose you could say: "You beautifies them there clothes baay-u-ti-fully, darlin'" Best not in an ESL exam though!