Antonym of UNSTABLE

Answer: Option D

Unstable : likely to give way; not stable.
Changing : not changing; remaining the same.
Constant : occurring continuously over a period of time.
Stagnant : (of a body of water or the atmosphere of a confined space) having no current or flow and often having an unpleasant smell as a consequence.
Steady : firmly fixed, supported, or balanced; not shaking or moving.

Antonym of Unstable is Steady