Final answer:
The term for when three asteroids are arranged in a line is called an alignment. This is an astronomical event that can occur within the plane of the ecliptic and affects gravitational forces. Therefore, the correct option is d. alignment.
Step-by-step explanation:
The geometric arrangement where three asteroids are in a line is called an alignment. In astronomy, an alignment refers to the configuration where celestial bodies such as asteroids, planets, moons, or the sun are positioned in a straight line.
This phenomenon can occur within the plane of the ecliptic, which is the imaginary plane containing the Earth's orbit around the Sun. Astronomical alignments are important because they can affect gravitational forces and are associated with events like eclipses, where the Moon, Earth, and Sun are aligned. However, in popular culture and science fiction, asteroid alignments are sometimes depicted as a potential hazard for space travel, even though, in reality, the vast distances between asteroids in the asteroid belt make such a navigational problem quite improbable.