Explanation

(b)
Random copolymer the polymer is made of two types of monomer units. The monomer units are arranged randomly. If $A\ and\ B$ are two different monomers, then random copolymer will have following structure.
$—A—B—A—\ B—B—B—\ A—A—A—B—$
Alternative copolymer the polymer is made of two types of monomer units arranged alternately eg.
$—A—B—A—B—A—B—A—B$
Cross-linked polymer in these types of polymers a short side chain of atoms links two longer linear chains of polymes.
Homopolymer it is polymer made of molecules of same substance e.g., polyethylene.