The correct answer is: B. True horizon
The true horizon is the plane that is tangent to the Earth’s surface at the observer’s position. It is the boundary between the Earth and the sky. The true horizon is always at eye level, and it appears to be a circle.
The rational horizon is a theoretical horizon that is calculated based on the observer’s latitude and the Earth’s radius. It is a circle that is tangent to the Earth’s surface at the observer’s position, and it is always at a right angle to the zenith-nadir line.
The celestial horizon is the apparent horizon as seen from the Earth. It is the boundary between the Earth and the sky as seen from a point on the Earth’s surface. The celestial horizon is always at eye level, and it appears to be a circle.
Therefore, the true horizon is the only option that is a plane at right angle to the zenith-nadir line and passing through the centre of the earth.