Triangle ABC is a right triangle, meaning we can use the Pythagorean Theorem to find x.
The formula for the Pythagorean Theorem is:

where a is the hypotenuse, and b and c are the legs.
In this problem, we have the vertical leg as 16, the horizontal leg as x, and the hypotenuse as 20. Therefore, we can say that

Therefore, we can plug into the formula to find x:

We first change the variable c to x and square both sides of the equation. Then we subtract b^2 from both sides.

We square root each side to find the variable answer for x. Then we plug in the numbers:

We find that x = 12. The answer is A. 12.