Thomas Paine, whose political writing was often flamboyant, was in private life a surprisingly (i) ________ man: he lived in rented rooms, ate little, and wore drab clothes.

Two ways of dealing with this sentence

Method 1

His political writing which was flamboyant is contrasted ("surprisingly") with his private life. So his private life has to be the opposite of flamboyant. From the choice list simple fits the best.

Method 2

The part after the colon explains, describes, defines the part before it. In fact the blank which describes him is just before the colon. And the part after the colon says "he lived in rented rooms, ate little, and wore drab clothes". Again simple is the best fit.