Thanks for the question! Let me start off with why (B) is wrong. (B) tells us that knowledge of our own thoughts is usually unmediated due to our expertise. But take a look at lines 25-27, and lines 35-38. In those lines, we’re told that there’s an illusion that happens to use when we become experts in a particular area. We think we can see and grasp entities and relations directly. But this isn’t true. Instead (lines 35-38), we become so expert in making fast introspective inferences about our thinking that we fail to notice that we are making them. In other words, knowledge of our own thoughts is mediated, not unmediated, we just don’t notice it. So (B) is wrong.
(C), on the other hand, is the correct answer because those same lines (and really, paragraph 2 in general) support it. (C) tells us that we are unaware of the inferential processes that allow us to become aware of our thoughts because we just become so good at doing it over time.
Hope this helps! Feel free to ask any other questions that you might have.