Answer:
Unacceptable.
Explanation:
The word "taboo" usually refers to anything that is deemed illegal, prohibited, repulsive, unacceptable, etc. It is used to talk of things that are not openly discussed, or accepted, and are secretive.
In the given scenario, Ron's refusal to talk about his salary shows he finds the topic unacceptable to be talked about openly. He seems to take the topic as something that is not openly talked about, and needs not do it. To him, his salary is something that is 'unacceptable' to be revealed.
