"Venal" means capable of being bought or influenced by money or other valuable considerations. 

"Avaricious" means having extreme greed for wealth or monetary gains.
"Mercenary" refers to professional individuals who join armed conflicts for their own profit.
"Untrustworthy" is an adjective describing someone negatively. 

"Incorruptible," on the other hand, is an adjective used to describe someone or something not subject to corruption or bribery. 

Of all the given options, "incorruptible" serves best as an antonym for "venal."

Therefore, Option D is the correct answer.