The sky’s very dark, it “looks like” there’s going to be a storm.
This option is correct because we aren’t 100% certain it will be a storm; however, telling from the cloud patterns and absence of light, we can deduct that it will likely rain heavily and storm.
Option A is incorrect because it makes a statement that there will absolutely be a storm, when in reality we are unsure.
Options B and C are both incorrect in that they’re comparing the current conditions to how it would look if it rained. In essence, both options present a likelihood that it won’t rain at all.
That leaves us with option D, which states a possibility that it will rain based on the conditions. This is the only accurate sentence filler.