Which of the following is an example of copolymer:
A. Buna-S
B. PAN
C. Polythene
D. PTFE

Hint: Copolymers have more than one type of repeating unit. Monomers of Buna-S are $1,3 - $butadiene and styrene. Monomer of PAN is acrylonitrile. Monomer of polythene is ethylene. Monomer of PTFE is tetrafluoroethylene.

Complete step by step answer:
When a large number of molecules join in a manner to form a long structure, the structure is known as a polymer. In a polymer, a molecule or unit repeats again, and again that’s why the formed structure is known as a polymer because the word ‘poly‘ means many, and ‘mer’ means units.
The units which repeat are known as repeating units or monomers.
Based on the type of polymer units the polymer is divided into two categories:
Homopolymer: When only one type of repeating unit is present in a polymer the polymer is known as a homopolymer.
Copolymer: When more than one type of repeating unit is present in a polymer the polymer is known as copolymer.
->The Buna-S is made up of two different types of monomer, $1,3 - $butadiene and styrene. So, the Buna-S is a copolymer, so option (A) is correct.
->The full name of PAN is polyacrylonitrile. PAN is formed by polymerization of monomers acrylonitrile. So, PAN is not a copolymer, so option (B) is incorrect.
->Polythene is formed by polymerization of monomers ethylene. So, polythene is not a copolymer, so option (C) is incorrect.
->The full name of PTFE is polytetrafluoroethylene. The PTFE is formed by polymerization of monomers tetrafluoroethylene. So, PTFE is not a copolymer, so option (D) is incorrect.

Therefore, option (A) Buna-S is correct.

Note:
The polymer PAN, polythene, and PTFE are homopolymer. The name of the homopolymer includes the name of the monomer unit from which the polymer is derived and the prefix ‘poly’.