Choice A is the best answer. In the context of describing the solar panel manufacturing industry as being "in the doldrums because supply far exceeds demand" (lines 2-3,"but the solar...demand"), saying it is currently a "poor" market implies it is a weak, or slow, market.
Choices B, C, and D are incorrect because in the context of describing the solar panel manufacturing industry as being "in the doldrums," saying it is a poor market implies it is a weak market, not a modest one (choice B), a pathetic one (choice C), or an outdated one (choice D).