What they're saying is that if you see a number in between brackets it is equal to integer that is just to the right of it on the number line. So

[-4.5] would equal -4

and

[4.5] would equal 5

so [-4.5] + [4.5] = -4 + 5 = 1

Now you have to see that all the answers still have the brackets around them, so you need to choose one that when using their rule would ultimately equal 1. Answer choice D [.5] would equal 1 because 1 is the least integer greater than .5