Correct Answer: Improper

Solution : The correct choice is the second option.

Appropriate means suitable or proper in a given context. The opposite of appropriate is something that is not suitable or not proper, which is represented by the word improper. Therefore, improper is the antonym in this context.

The meanings of the other options are as follows:

• Transparent : It means allowing light to pass through so that objects can be seen through the substance.
• Salient: This word means most noticeable or important.
• Disturbing : This word means causing anxiety or unease.