Correct Answer: Opacity

Solution : The right choice is the fourth option.

Explanation: The term obliquity refers to the quality of being obscure, unclear, or indirect in speech or action.

Similarly, the term opacity refers to the quality of being unclear, obscure, or difficult to understand. Both terms relate to a lack of transparency or straightforwardness.

The meanings of the other words are:

• Profundity: It refers to depth, especially intellectual or emotional depth. It also indicates deep insight, knowledge, or significance.
• Incisiveness: It refers to the quality of being clear, direct, and sharp in thought or expression.
• Surety: It refers to certainty or confidence in something. It can also relate to a person or entity that provides a guarantee or assurance for the fulfilment of an obligation.

Hence, the most appropriate synonym is opacity.