The correct option is A Buna-S
A copolymer is a polymer made out of 2 or more kinds of monomer units.
In the options gives, only Buna-S is made out of butadiene and styrene making it a copolymer. Usually, when the term 'poly-' is part of the name, the polymer is a homopolymer like polyethene or polyester.
Note - Don't confuse copolymer with a condensation polymer.