The correct statement that explains why momentum is conserved in this situation is:
C. The net force acting on the system is zero.
In an isolated system where no external forces are acting, the total momentum remains constant. Before the spring is released, the momentum of the system is zero (assuming both carts are initially at rest). After the spring is released, the two carts push apart with equal and opposite momenta, which keeps the total momentum of the system at zero, thus conserving momentum.