"Submissive" means ready to conform to the authority of others; meek, obedient, or docile.
Checking options:
- (a) Obstinate → opposite (stubborn).
- (b) Unyielding → opposite of submissive.
- (c) Stubborn → opposite meaning.
- (d) Docile → correct synonym, meaning obedient and easily managed. \[ \boxed{\text{Docile}} \]
Group the options by whether they describe resistance to authority or compliance with it, since "submissive" specifically describes yielding to others' will.
Three of the four options describe resistance to authority, while only "Docile" describes compliance, matching "submissive."
Hence, the correct answer is Docile.