Correct Answer: Unconvincing

Solution : The correct option is the second option.

Explanation:
Cogent means convincing or persuasive, having the power to compel or convince someone. So, the opposite, unconvincing, would be the antonym in this context, as it signifies lacking persuasive power or failing to convince.

The meanings of the other options are as follows:

• Compelling means having a powerful and irresistible effect.
• Pertinent means relevant or applicable to a particular matter, which is related but not an antonym of "cogent".
• Unexpected means not expected or surprising, which doesn't directly relate to the persuasiveness or quality of the reasons given.