Correct Answer: Cynical

Solution : The most appropriate choice is the third option.

The word gullible means easily fooled or deceived, often because one is too trusting or naive. Cynical is the most appropriate antonym because it reflects a sceptical and distrustful attitude, which is opposite to the trusting and easily deceived nature of someone who is gullible.

The meanings of the other options are as follows:

• Credulous: It also means being too willing to believe or trust in things without questioning or being sceptical.

• Pliant: Easily bent, flexible, or adaptable.

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• Simple: It means uncomplicated or easy to understand.