The correct option is B 4k
The restoring force is constant and is independent of the length of the spring.
Using this concept, we have, if k2 is the spring constant of the cut spring of length (L/2), then F=kL=k2(L/2)⟹k2=2k
If these two springs are joined together in parallel, their spring constants gets added up. Thus effective spring constant is keff=4k
The correct option is (b)