Answer:
Axis of symmetry.
Explanation:
We have been given an incomplete statement. We are supposed to complete the given statement.
Given statement: The vertical line passing through the vertex of a parabola is called the ________.
We know that a parabola is symmetric about axis of symmetry . The line passing through the vertex of parabola divides the parabola into two mirror images.
Therefore, the vertical line passing through the vertex of a parabola is called the the axis of symmetry.