Let's look at all the options first. All of the options are below 7 and 10, hence we can deduce that it is not the hypotenuse. Looking at 7 and 10, we see that 10 is the larger number, and hence we know that 10 is the hypotenuse.
Hence, as the other two sides added up in a triangle must be larger than the hypotenuse, the only available option is 4, since 4 + 7 = 11 > 10.
