The correct option is A moving
The word 'stationary' refers to something not moving or intended to be moved.
Option A, moving refers to something in motion.
Option B, speeding refers to something that is moving quickly.
Option C, shunting refers to pushing or shoving something.
Option D, standing refers to something stagnant or still.
Thus, according to the meaning, the word 'moving' is the opposite of the word 'stationary'.