Answer
A plane, passing through all points on the y-axis, that cuts through the xz-plane at a 45 degree angle.
Work Step by Step
Using intuition, create a two-space coordinate plane xz. For every value x, z must be equal. So, on our 2D coordinate plane, we end up with a line that leaves the origin with a slope m=1.
In three-space, our 2D coordinate plane is now our xz-plane. Our 2D line transforms into a plane to accommodate all values of y. What was our 2D slope, m=1, can now be seen as all points on the plane meeting the x-axis at a 45 degree angle relative to the origin.
