"They are your socks."
Okay, so all this time sus meant your, and was acceptable, and now all of a sudden it is not?
Su (singular) /
Sus (plural) can mean "your" when you are being formal:
Son sus calcetines can be rewritten as any of the following:
Son los calcetines de Usted.= "They are your (formal) socks."
Son los calcetines de él.= "They are his socks."
Son los calcetines de ella.= "They are her socks."
Son los calcetines de ello.= "They are its socks."
In conversation, you would rely on the context to figure out the meaning of