答案：B

解析：

The graph in the xy -plane of the quadratic function f(x)=x*x-6x+8 is a parabola. If the graph of the line with equation y=a intersects the graph of this parabola in exactly one point, that point must be the vertex of the parabola, and the Y-coordinate of the point must be a. The graph of f(x)=x*x-6x+8=(x-2)(x-4) intersects the x -axis at x=2 and x=4, so the x-coordinate of the vertex of the parabola is halfway between x=2 and x=4 on the x -axis at x=3. Thus the y -coordinate of the vertex is f(3)=9-18+8=-1. Therefore, if the graph of the line with equation y=a intersects the graph of the quadratic function f(x)=x*x-6x+8 in exactly one point, the value of a must be -1.