Dear Zea,
After receiving your letter and knowing the problem you are facing. Today, I will give you some advice in choosing the right pet and how to persuade your parents to give you one.
In my opinion, cats will be the suitable pets for you to keep in your apartment. You can keep cats like: Siamese cats, tabby cats, etc
In case your parents are not interested in letting you have a pet, you can tell them that having a cat is like having a friend to improve your mood, relieve stress after studying hard. Besides, you also have to promise them that you will be responsible for the cat you raise. You also need to learn how to make your parents stop worrying about the negative things about having a cat, such as bad smell or dirt, etc.
Above is my suggestions for the problem you are facing. I hope you will find the suitable pet and persuade your parents to let you have one. 

Sincerely,

Dung