Propene is an alkene. The structural formula of propene is shown. A polymer can be made from propene. What is the correct structural formula of this polymer?

Answer:

18) D

In this polymerization reaction, the double bond breaks down into a single bond and the individual propene molecules (monomers) combine to form a long, chainlike molecule consisting of repeating units.
Imagine the given propene like this (the numbers in red are the carbon number in propene):
You can see that there is a double bond between the first and second carbon. This double bond would break, forming something this:
Carbon atoms usually need 4 bonds in total to be stable. Since the initial double bond broke into a single bond, they both need one more bond each. This is when they combine with other propene molecules and form repeating units. The ‘n’ outside the bracket shows that there are ‘n’ number of repeating units in the propene polymer.

D is the correct answer.