Work Step by Step
RECALL: (1) The graph of the quadratic function $y=ax^2+bx+c$ is a parabola that opens: (i) downward when $a \lt 0$; or (ii) upward when $a \gt 0$ (2) A parabola that open upward has the vertex as its minimum point, while a parabola that opens downward has the vertex as its maximum point. The given function has its maximum value at its vertex. This means that the parabola opens downward. Thus, $a \lt 0$. The answer is Option (d).