In its search for new technologies for solar energy that can produce

­Your concerns are valid, and (C) certainly does not PROVE that Smith-Diatom will be able to produce low-cost electricity from dye-sensitive solar cells. But we don't need proof -- we're only looking for the BEST answer choice. In this case, we want the answer choice that most strongly supports the hypothesis that the plan will work.

The passage talks about production of electricity using diatoms and how it would be more efficient than current methods. So if you have one system using diatoms and another using the older method, the one using diatoms will cost less to run (i.e. less money per unit of electricity created).

But what about the cost of actually building and maintaining that system? The passage doesn't talk about that at all. If you need to spend a ton of money to make the new dye-sensitive solar cells, then the savings (thanks to the improved efficiency) might not outweigh the costs.

(C) eliminates that possibility and thus strengthens the hypothesis. It's not necessarily a very satisfying OA, but it's certainly better than the other four.

I hope that helps!