Hamlet feels that is where a woman has a best chance at being faithful, and where she will cause the least amount of damage. After all, as he tells Ophelia also, "why wouldst thou be a breeder of sinners?"; in a nunnery, she won't have children and bear wicked men-like his uncle-that do awful things.
The last option. "Heather does not usually turn in her homework on time." This can be gathered from her statement of "I finally did my homework," indicating she usually doesn't, as well as her teacher saying "again?" meaning this is a reoccurring thing.
