The equilibrium position of an object in an oscillating system is always the point where

(A) x = 0. (B) v_x = 0.

(C) a_x = 0. (D) p_x = 0.

The correct option is (A).
The particle moves toward its equilibrium position at x = 0. The force must always act to restore the particle to its equilibrium position. Therefore the equilibrium position of an object in an oscillating system is always the point where x = 0. Thus option (A) is correct.