Answer:
C. 
Step-by-step explanation:
We have been given a function formula
. We are asked to find the axis of symmetry for the given function.
We can see that our given function is a parabola in vertex form. We know that vertex from of parabola is
, where (h,k) represents vertex of parabola.
Let us convert our given function in vertex form as:

We know that axis of symmetry of a parabola is the equation of x-coordinate of vertex, which in form
.
Since the x-coordinate of vertex is
, therefore, the axis of symmetry for the given function would be
.