the meaning of malice is 'to harm someone/something'
So, based on this meaning, we can derive the structure of the passage as follows:
Don't harm someone (or something) --> because harming = stupidity.
Roommate knew that he's not supposed to water the plant more than once a week
Therefore, by over-watering, he (the roommate) intended to kill my cactus plant. One possible way to weaken the argument is for us to weaken the idea that the roommate had the intention to kill the plant; in other words, we need to show that the roommate didn't intend to kill the plant at all - maybe he was genuinely trying to help.So, lets find the answer choice that weakens the argument
A.
Intending to destroy a plant is a form of malice. - this is actually an assumption that has led to the conclusion of the argument; knowing what (A) says does not weaken the argument in any way. So,
eliminate (A)
B.
The roommate resented being obliged to water the cactus. - Okay. the roommate may have hated having the responsibility to water the cactus. But how does this weaken the idea that 'he (the roommate) intend to do so? This isn't clear. So,
Eliminate (B)C.
The roommate expressed great sorrow upon being told that he was responsible for the death of the cactus. - What if the roommate did have the intention to kill the cactus plant? After doing so, he might play innocent and claim that he was sorry for doing So. This statement can be used to interpret the argument in a very different way: You could also say that the roommate truly didn't intend to kill the cactus plant. Since (B) can be interpreted in both ways, it does not weaken the argument 100%. So,
eliminate (C)D.
The roommate is a member of a local botanical society. - This has the same problem that (B) has - (D) does not weaken the question at all. In fact, (D) is irrelevant. Hence,
eliminate (D)E.
The roommate was unaware of the amount of water the cactus was to be given. - In other words, the roommate did water the cactus but was unaware of the amount of water that it had to be given. So, he (the roommate) has made an unintentional mistake of over-watering the plant. This certainly rules out (or weakens) the author's claim that the roommate 'intended to kill the cactus'.
Hence, (E) is the right answer.