Formal style, it's pretty easy to figure that out by just canceling out the other answers. Slang is obviously not it, I don't see anything critical in this, and I don't know how it could be informal.
-tion is used to form NOUNS from verbs.
Answer:
On the lush tropical island, exotic fruits and flowers grew in abundance
Explanation:
I think exotic would be the best choice in the sentence because if you're on a tropical, you should find wild animals and exotic fruits, and abundance would also be the best choice because also in a tropical island, you would also find lots of different kinds of flowers.
