I agree with your answer. It is choice C.
This is because the event "selling items" leads directly to the event "earning money". Keep in mind that this is the revenue and not the profit, so you'll have to account for expenses of course.
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Something like choice A may seem to be an example of causation, but traffic accidents happen in other situations as well such as rainy weather or distracted driving. We can rule out choice A.
Choice B isn't the answer either because you may have a really skinny and tall person.
Choice D is likely to have strong positive correlation (since people are probably using wood stoves more when it gets cold), but the cold weather itself on its own is not causing the fires. Instead, the actual fire material is doing that. So we can rule choice D out as well.
