The correct option is
D Unsure
The word 'confident' refers to the feeling or showing confidence in oneself or one's abilities or qualities.
Option A, vague refers to an uncertain, indefinite or unclear character or meaning.
Option B, cowardly refers to the feeling of lacking courage.
Option C, secretive is characterized by the concealment of intentions and information.
Option D, unsure refers to not feeling, showing, or done with confidence and certainty. Thus, according to the meaning of the given word, this will be the correct antonym to 'confident'.