The correct option is A descent
The word 'ascent' refers to a rise to a higher social or professional rank.
Option A, decent refers to being satisfactory and of an acceptable standard.
Option B, descent refers to the origin or background of a person in terms of family or nationality.
Option C, descant refers to talking tediously or at length.
Option D, descendent refers to descending from an ancestor.
Thus, according to the meaning, the word 'descent' is the opposite of the word 'ascent'.