Thanks Aadika. Here's my thought process on how I approached it.

Reading the question "Hanlon’s Razor cautions that one should never attribute to malice that which can instead be attributed to stupidity. But my roommate knew that my prize cactus should only be watered once a week, so it is clear that by overwatering the plant he intended to destroy it." - so hence to weaken the argument there must be some stupidity involved in watering the plant or some incompetence of sorts.

Going through the arguments....

Intending to destroy a plant is a form of malice - Doesn't weaken argument, somewhat irrelevant, bin
The roommate resented being obliged to water the cactus. - Irrelevant, bin it
The roommate expressed great sorrow upon being told that he was responsible for the death of the cactus. - Irrelevant, talking about intentions rather than stupidity, not direct link
The roommate is a member of a local botanical society.- Irrelevant
The roommate was unaware of the amount of water the cactus was to be given.- Yep. This one implies stupidity and ignorance. If roommate was aware of amount of water the cactus would be given, this would strengthen the conclusion. So this one is right.

Hope it helps.