1) Lysosomes, because they are the organelles that secrete these hydrolytic enzymes and help break down old organelles, viral/bacterial invaders, and food particles. For more information on cells please see the link in sources, it's a great slide show and can probably help you in the future too. Bookmark it!
2) There are some really great sites explaining homeostasis and active transport below in the sources, some with animation that may make it a little easier for you to understand exactly what happens.
3) If you understand active transport first, the answer will actually be very easy. All you have to do is explain how active transport maintains homeostasis in the first place, then apply it to the specific situation of the fish. I honestly feel you need to answer this one yourself, since it is very clearly a question to test the comprehension of this material on your part; if you don't get it, you probably won't pass, and you'll be better off in the long run answering it yourself.
I hope the links help! And don't be afraid to use your book's index to find more information in chapters you haven't covered yet!
