The __(X)__ energy of the spring is said to be zero when block and spring system is in the __(Y)__ position. Here, X and Y refer to :

kinetic, equilibrium  potential, equilibrium mechanical, equilibrium  vibrational , left

potential, equilibrium

In equilibrium position i.e. the mean position, the displacement from the mean position is zero. Thus, the potential energy given by : \(\frac{1}{2}kx^2\) where x is the displacement is zero.