The correct option is
B Step-growth polymer
Step-growth polymerization refers to a type of polymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers, then trimers, longer oligomers and eventually long chain polymers. Bakelite forms by the polymerization of phenol and formadehyde. It forms cross links between different chains of the polymer and thus forms by step-growth mechanism.