This might could help
http://diy.stackexchange.com/questions/998/what-are-the-different-gauges-of-electrical-wire-availabl...
I'd say D. expert power because expert power is when an employee ( or in your case the teacher) has a high set of knowledge/skills that other employees do not posses. Which by the information above about this teacher's PhD and thier experience, I assume that this teacher must have expert power.