Concept:The missing word must describe a person who easily believes lies because of inexperience or poor judgment.
Explanation:"Naive" means showing a lack of experience, wisdom, or judgment.
A naive person can be easily deceived, so believing his lies fits the context.
"Credible" means believable or trustworthy, so it does not describe the person.
"Childish" suggests immature behaviour, which is not the same as being easily fooled.
"Modest" means humble, and it is unrelated to believing lies.
Therefore, the best completion is: "Don't tell me you are naive enough to believe his lies."
Answer:D. naive