Answer:
C. x² -8x +16 = 20
Explanation:
You want the next step in completing the square after you get x² -8x = 4.
Completing the square
The constant in a perfect square monic trinomial is the square of half the coefficient of the linear term.
The coefficient of x is -8. Half that is -4, and its square is (-4)² = 16. This will be the value added to the equation in the second step.
x² -8x +16 = 4 +16
x² -8x +16 = 20
__
Additional comment
A monic polynomial has a leading coefficient of 1. We generally work with monic quadratics when completing the square. We do this by factoring the leading coefficient out of the variable terms.