They both mean pretty much the same thing. "Stockings", "socks", "pull-ups", etc, I've seen them all being used to mean "sock".

I wrote "I suoi calzini" and got it right, the other translation will be "Le sue calze". I ever heard that "I suoi" ia masculine while "Le sue" is for feminine form

I put "I suoi calzini" as well. The article (i) and the possessive (suoi) must match the noun (calzini) in gender and plurality. If you were translating "his sock" (singular) it would be "il suo calzino."

I learned the plural of sock as calzini also.

It Probably Depends How You See The Words. I Mostly Use Stocking To Refer To An Oversized Sock I Hang Up For Christmas, And When Not Referring To That I Just See It As A Synonym Of Sock.

That would be "your socks".

• le sue calze = his/her socks
  le Sue calze = Your socks (if you use "Sue" as formal your)

• his socks = le sue calze

The only time singular vs plural is relevant is for family members. You don't use the definite article with singular family members.

Perché in italiano devi sempre mettere l'articolo