B.
A is ethylene and chlorethene (vinyl chloride). If you're lucky you could make an addition polymer out of it but there is definitely no way to expel a small molecule as required in condensation polymerisation.
C brings chloethene back with another thing. That second thing actually looks like it can partake in a condensation polymerisation reaction.
Similarly D brings back ethylene
But if you look at those molecules in C and D, they don't really allow for a site for condensation polymerisation to occur. It's not likely to expel an H2 molecule in condensation polymerisation and then just allow the C to form a bond with the O to make your dimer.
B gives you the more feasible opportunity to expel an H2O molecule instead and then form the C-O bond. By inspection this is far more favourable - the expelled molecule can be a liquid instead of a gas.