Luminous means something that shines in the dark.
So the opposite/ antonym of it is option A which is ‘dark’ as dark means very little or no light, hence it is correct.
Option B is incorrect because ‘ludicrous’ means silly or ridiculous which is not in any way related to light, dark or luminous.
Option C is incorrect because ‘unsteady’ means gaming or moving in such a way that is not controllable, hence it is incorrect.
Option D is incorrect because ‘provoking’ means the act of producing a particular feeling or reaction, hence it is incorrect.