He is adamant on going.

Answer: Option D

Adamant : refusing to be persuaded or to change one's mind.
Confused : (of a person) unable to think clearly; bewildered.
Lenient : (of a punishment or person in authority) more merciful or tolerant than expected.
Stressing : give particular emphasis or importance to (a point, statement, or idea) made in speech or writing.
Yielding : give way to arguments, demands, or pressure.

Antonym of Adamant is Yielding