When considering principled negotiation, keeping focus on the issues even when people become upset, frustrated, and even angry is an example of?

Separating people from the problem is the correct answer.

In the business world, principled negotiation focus on finding win-win solutions while protecting yourself from those who want to take advantage of your forthrightness. Negotiating can be very difficult when there's no mutual gain and it's natural that different issues come up. However, when considering principled negotiation, it's important to focus on the issues (regardless of how upsetting and frustrating it can be) and separate people from the problem.