Answer:
D.
Step-by-step explanation:
After the third step, take the square root of both sides.
Third Step:
[tex](x+\frac{b}{2a})^2=\frac{b^2-4ac}{4a^2}[/tex]
Thus, the fourth step is:
[tex]\sqrt{(x+\frac{b}{2a})^2}=\sqrt{ \frac{b^2-4ac}{4a^2}} }[/tex]
The left side cancels:
[tex](x+\frac{b}{2a})=\pm\sqrt{ \frac{b^2-4ac}{4a^2}} }[/tex]
D.
