The correct option is D Butadiene
Ethene is C2H4, Styrene is Ph−CH=CH2, Isoprene is CH2=CH−C(CH3)=CH2 and Butadiene is CH2=CH−CH=CH2
All of them are small molecules and not polymers, rather they are monomers.
Hence, all the options "A", "B", "C"&"D" are correct answers.