9427178547B and C are connecting very different concepts. The issue is less in what's different between B and C, and more in what is missing from the argument.
First, notice that the conclusion is in the middle, followed by another premise introduced by SINCE. Each of the statements is conditional (if this, then that), so I'll show that connection with arrows.
Premises:
Don't think people mistrust --> Confident in abilities
Confident in abilities --> See difficulty as challengeNotice that one premise leads right into another, like this:
Don't think people mistrust --> Confident in abilities --> See difficulty as challengeSo by combining the premises, we already know that people who don't feel mistrusted will see difficulties as challenges.
However, what does the conclusion say?
Trust others --> See difficulty as challengeWe don't know anything about trusting others! We just know about people who don't think others mistrust them. So we need to connect those. We need this:
Trust others --> Don't think people mistrustThat's exactly what C says. With that assumption in place, we have a complete logical chain that fits the conclusion:
Trust others --> Don't think people mistrust --> Confident in abilities --> See difficulty as challengeB, on the other hand, gives us something entirely different.
Confident in abilities --> Confidence in trustworthiness of others. That's nothing like what we're looking for.