Buna-N synthetic rubber is a copolymer of :

The prefix “Buna” tells us that the polymer contains $$Bu$$ = butadiene and is prepared by $$Na$$ = sodium-catalysed polymerisation.
The second letter that follows (“S”, “N”, etc.) identifies the comonomer.

For Buna-N the identifying letter “N” stands for nitrile (more precisely, acrylonitrile or vinyl cyanide, $$CH_2=CH-CN$$).

Hence Buna-N is obtained by copolymerising:
• $$1,3$$-butadiene, $$CH_2=CH-CH=CH_2$$
• acrylonitrile, $$CH_2=CH-CN$$

Therefore Buna-N synthetic rubber is the copolymer of $$1,3$$-butadiene and acrylonitrile.

Option B which is: $$1,3\text{-butadiene + acrylonitrile}$$