The correct option is C 4k
Spring constant is inversely proportional to length of the spring. i.e k∝1l=c1l where c is proportional constant.
so when it is cut into two equal half, spring constant each half, k′=c1l/2=2cl
thus, k′k=2⇒k′=2k (Basically there are now half as many coils to take up the force via displacement, thus spring constant increases)
As the two half are connected in parallel so the constant of the combination is knet=2k+2k=4k