The correct option is C bilateral
The word 'unilateral' refers to involving only one group or country.
Option A, collective refers to something done by people acting as a group.
Option B, bilateral refers to something involving two parties, especially countries.
Option C, multilateral refers to involving three or more parties.
Option D, multiple refers to involving several parts, members or elements.
Thus, according to the meaning, the word 'bilateral' is the opposite of the word 'unilateral'.