The correct option is
C Darken
Antonyms are words that have contrasting, or opposite meanings.Choice B is correct. The word 'illuminate' means 'light up' the opposite is 'darken', which means 'make or become dark'. For example: The room was light up with candles, but it was still dark.
Choice A means 'give someone knowledge' Choice C means 'expose to radiation' Choice D means 'think back'. Choice A, C and D are wrong.