The correct option is D cowardly
The word 'dauntless' refers to showing fearlessness and determination.
Option A, devious refers to being deceitful and dishonest.
Option B, subtle refers to being delicately complex and understated.
Option C, secret refers to something confidential.
Option D, cowardly refers to lacking courage.
Thus, according to the meaning, the word 'cowardly' is the opposite of the word 'dauntless'.