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I was originally leaning towards C as well but the sentence seems to say tha the forms of solar energy will be economically competitive now or in a few years. The "will be" in the sentence tells me that the forms of solar energy are not yet economically competitive.
D works for me because it states that forms of solar energy are either (a) already economically competitive or (b) will be in a few years.
Check out this awesome article about Anderson on Poets Quants, http://poetsandquants.com/2015/01/02/uclas-anderson-school-morphs-into-a-friendly-tech-hub/ . Anderson is a great place! Sorry for the lack of updates recently. I...