The line of symmetry is vertical line that passes through the vertex.
Find vertex of this parabola has x-coordinate:
[tex]x_v=-\dfrac{b}{2a}.[/tex]
Thus,
[tex]x_v=-\dfrac{8}{2a}=-\dfrac{4}{a}.[/tex]
Then the line of symmetry has the equation
[tex]x=-\dfrac{4}{a}[/tex]
and coincides with the line [tex]x=4.[/tex] This means that
[tex]-\dfrac{4}{a}=4,\\ \\a=-1.[/tex]
Answer: correct choice is B
