Which is the only planet that is tilted on its axis?
They are all tilted.
What planet rotates around an axis tilted at 90 degrees Celsius relative to the plane of the solar system?
Tilted compared to what? The Earth itself is mostly round, and with all the mountains, is hardly flat. Most surfaces are tilted compared to the Earth itself. The Earth spins on its axis, and the axis of rotation is only "tilted" when compared to something else, such as the plane of the Earth's orbit around the Sun. (We call this plane the "ecliptic".) The Earth's axis of rotation is tilted about 23.5 degrees to the…