The correct option is D Isoprene
Natural rubber is an addition polymer that is obtained as a milky white fluid known as the latex from a tropical rubber tree. Natural rubber is from the monomer isoprene (2-methyl-1,3-butadiene). Since isoprene has two double bonds, it still retains one of them after the polymerization reaction. Natural rubber has the cis configuration for the methyl groups. So, the correct option is A i.e. Isoprene.