Option ‘a’ rotten means stricken by decay. This word may be a distractor because it isn’t opposite in desiring to genuine. Therefore, it's an incorrect answer.
Option ‘b’ bogus means to be fake. This word is opposite in aiming to genuine. Therefore, it's the right answer.
Option ‘c’ unsound means to be in an exceedingly poor condition. This word may be a distractor because it isn’t opposite in aspiring to genuine. Therefore, it's an incorrect answer.
Option ‘d’ impure means to be mixed with foreign matter. This word could be a distractor because it isn’t opposite in desiring to genuine. Therefore, it's an incorrect answer.
So, the proper answer is “Option b”.