1. The passage claims that the existence of fire damage to houses whose roofs were doused signifies that dousing the roofs was absolutely useless - in other words, A WASTED EFFORT. Therefore, in order to weaken this conclusion, you have to find an option that shows that dousing the roofs was effective to at least some degree.

2. You say that you eliminated option A because it contains a comparison that is not in the passage. But the question is not whether the comparison is in the passage, but rather whether the comparison will have some impact on the conclusion. It can do this only if it is RELEVANT. And since option A establishes that more fire damage was suffered by those who did not douse their roofs that by those who doused them, the comparison is in fact relevant: it shows that dousing lessened the fire damage, and was therefore NOT entirely useless - i.e. NOT a wasted effort.

3. Now either I am having an off day and something is escaping me, or option D is also correct. If that's the case, this is not an ideal practice question. If remaining in the house and dousing the roof means that an owner's home is going to suffer less damage - THE LEAST DAMAGE, in fact - then dousing the roof is not entirely useless - it is, in fact, fairly efficacious, and therefore decidedly NOT a wasted effort.
If you try the Negation Test on option D, and replace LEAST with MOST, it will be clear that continuously dousing the roof was indeed useless, so strengthening the conclusion that dousing was A WASTED EFFORT. Thus, in its original form, option D weakens the conclusion - or I'm blind Freddy.

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