The word venal means to be susceptible to corrupt bribery. While reading this, I see the man is obviously innocent, but his character is being judged, not his actions. He is disheartened, so "serious" and "conceited" are out of the question. "Lacking originality" could be a potential answer because he is a poor man on trial, but D, easily corrupted is the correct answer because it is most in line with Randall's current situation.