Answer:
Increases should be the word.
Step-by-step explanation:
When we are speaking in the present tense, in this sentence, we will add "s" to the end, making the word a verb. This particular word is telling you that in the future, the world population will increase at a rate of 2.5%. However, speaking in present-tense, we should notice that "will" is not in this sentence, therefore, the word should be increases.
Here's what the sentence should look like: "The world population increases at a rate of 2.5%."
I hope this helps!
-No one