Here's what's happening (the ladder is blue)


Since the wall is perpendicular to the ground we can use the Pythagorean theorem to find the value of x
We get: x² + 12² = 20²
Simplify: x² + 144 = 400
This means: x² = 256
So, x = 16
Answer: C


Alternatively, if you recall the Pythagorean triple 3-4-5, you might recognize that the two give sides of our diagram are each 4 times the values in the Pythagorean triple.
In other words 20 = 4(5), and 12 = 4(3)
So, we can conclude that x = 4(4) = 16
Answer: C