The correct choice is :
The final momentum of the bullet/block is equal to the initial momentum of the bullet.
initially the block was at rest , hence
initial momentum of the block = 0
so total initial momentum was simply equal to initial momentum of bullet only.
after collision , the bullet gets embedded in the block , and hence both block-bullet move together at common velocity . according to conservation of momentum , momentum remains conserved in any type of collision.
so final momentum of bullet-block system = initial momentum of bullet