Here, we need to find the flaws in the given argument reasoning.
Premise: Hanlon's prize cactus should only be watered once a week.
Conclusion: Having been told that the prize cactus should be watered once a week, overwatering the plant is an intention by the roommate to destroy it.

OPTION A - INCORRECT
This doesn't weaken the argument because according to the passage, intending to destroy a plant is a form of malice supports the argument.

OPTION B - INCORRECT
If true, the argument is not concerned with whether or not the roommate resented his obligation to water the cactus.

OPTION C - INCORRECT
The remorseful expression of the roommate proved Hanlon's statement right about overwatering the cactus.

OPTION D - INCORRECT
This option is wrong because it wasn't stated in the argument if the roommate was from the local botanical society. Therefore, it doesn't weaken the argument.

OPTION E - CORRECT
This option weakens the argument because Hanlon declared in the argument that the roommate was aware that the prize cactus should only be watered once a week.