It is actually C, Solar Power. "Land," as PLATO wants you to learn it, is essentially whatever natural resources are available to you in a given region when constructing something there. This includes things which aren't traditionally thought of as land, such as mineral veins in the area, fertile soil, and yes, solar power. Very tricky question.
For reassurance, I got 5/5 on the mastery test and I answered Solar Power, so it must be correct, even if it seems strange.