Concept:Salient means most noticeable or most important.
Solution:"The salient points of the topic" are its main, key points.
Replacing the word: "The important points of the topic have been thoroughly discussed" keeps the meaning.
A. "highlighted" means marked out for attention; points can be highlighted without being the main ones.
B. "chosen" means selected, which says nothing about importance.
D. "interesting" means attracting attention or curiosity, not being key.
Key point: "Salient" is close to "main", "key" and "chief".
Answer:Option C (important)