Let us first define line of symmetry of parabola :
It is the line of symmetry of a parabola that divides a parabola into two equal halves that are reflections of each other about the line of symmetry. It intersects a parabola at its vertex. It is a vertical line with the equation of x = -b/2a.
So using the formula x=-b/2a, we can find the line of symmetry of parabola here.
We are given the equation:

If we compare it with the quadratic equation :

we get a=1, b=-10 and c=21
Now plugging the values of a and b in the formula x=-b/2a,


x=5
So the line of symmetry of given parabola is given by x=5
Option D is correct answer.