Definition: A function is "even" when f(x) = f(−x) for all x.
Geometrically speaking, the graph face of an even function is symmetric with respect to the y-axis, meaning that its graph remains unchanged after reflection about the y-axis.
If point (6, 8) is one the points on the graph of f(x), then f(6)=8 and since function is even, you can state that f(-6)=f(6)=8. This means that point (-6,8) must also be a point on the graph. Geometrically it means that the output of a negative x-value and its opposite is the same.
Answer: correct choice is A.