Always be impartial in your dealings.

Answer: Option D

Impartial : treating all rivals or disputants equally.
Discourteous : showing rudeness and a lack of consideration for other people.
Impudent : not showing due respect for another person; impertinent.
Disrespectful : showing a lack of respect or courtesy; impolite.
Unfair : not based on or behaving according to the principles of equality and justice.

Antonym of Impartial is Unfair