The correct option is D descent
The word 'ascent' refers to an upward slope or path that one may walk on or climb.
Option A, slide refers to a smooth sloping surface.
Option B, decline refers to moving downwards.
Option C, fall refers to moving from a higher to a lower level, typically rapidly and without any control.
Option D, descent refers to an act of moving downwards.
Thus, according to the meaning, the word 'descent' is the opposite of the word 'ascent'.