The correct answer is B. Horizon.
The altitude of a heavenly body is its angular distance, measured on the vertical circle passing through the body, above the horizon. The horizon is the line that separates the sky from the Earth. It is an imaginary line that is always at eye level.
The altitude of a heavenly body can be measured using a sextant or a theodolite. A sextant is an instrument that is used to measure angles in the sky. A theodolite is an instrument that is used to measure angles in the horizontal and vertical planes.
The altitude of a heavenly body can also be calculated using trigonometry. Trigonometry is a branch of mathematics that deals with the relationships between the sides and angles of triangles.
The altitude of a heavenly body is important for navigation. It can be used to determine the latitude and longitude of a location. It can also be used to determine the time of day.