The correct option is A dispersed
The word 'gathered' refers to coming together, assemble or coming together.
Option A, collected refers to something brought together in one.
Option B, dispersed refers to being distributed and spread over a wide area.
Option C, spread refers to extending over a large area.
Option D, disappeared refers to ceasing to be visible.
Thus, according to the meaning, the word 'dispersed' is the opposite of the word 'gathered'.