The correct option is
C Division
The word 'unity' refers to the state of being united or joined as a whole.
Option A, bias refers to an inclination or prejudice for or against one person or a group.
Option B, diversity refers to a range of different things.
Option C, division refers to the action of separating something into parts.
Option D, weakness refers to a disadvantage or fault.
Option E, din refers to a loud, prolonged and unpleasant noise.
Thus, according to the meaning, the word 'division' is the opposite of the word 'unity'.