Answer: A
Step-by-step explanation:
We will start with process of elimination: This is a hyperbole, not a metaphor. This cannot be a metaphor, because the writer is NOT comparing anything. This is a hyperbole because the writer is exaggerating about the amount of water; it wouldn't cover the WHOLE world. Knowing that, we can rule out B and C!
Therefore, the answer must be A or B. If the rain was "torrential and constant", the author would have most likely used different words to convey that, such as "the water was unrelenting and bore down on us, seeming to engulf not just us, but the whole world". The sentence from the passage talks about the AMOUNT of water, not the way it came down.
Therefore option A is the best fit, as it talks about how the author cannot believe how much water there is. There would certainly be a lot if it "seemed to engulf the world".
Hope that helped!! :)