This is basically proving the quadratic formula. B is the correct answer.
Here is what I did after step 5:
x²+bx/a+b²/4a² = b²/4a² - 4ac/4a²
(x + b/2a )² = b²-4ac/4a² (The answer)
√(x+b/2a)² = + or - √b²-4ac/4a²
x+b/2a = + or - √b²-4ac/2a
x = -b/2a + or - √b²-4ac/2a