The answer you are looking for is C. 8.
To find this you could Pythagorean Theorem. The base or bottom of the triangle would be 7, and the "leg" of the triangle would be 3. In this instance, we will call 3 "a" and 7 "b". We use these two numbers to find the missing value, which is "c" or the hypotenuse.
The formula for Pythagorean Theorem is "a^2 + b^2 = c^2". Then we follow these steps.
Here's a picture to show how I did this. I hope this helps!