Final answer:
The highest point on the graph of a parabola that opens downwards is the vertex of the parabola.
Step-by-step explanation:
The highest point on the graph of a parabola that opens down is the vertex of the parabola. In a parabola that opens downward, the vertex represents the maximum point. The axis of symmetry is a vertical line that divides the parabola into two mirror-image halves. The domain of a parabola refers to all the possible x-values, while the range of a downward-opening parabola includes all the y-values up to and including the y-coordinate of the vertex. Therefore, the correct answer is b. vertex.