Which characteristic describes a planet's mass requirement?

The requirement that a planet must have sufficient mass to assume a hydrostatic equilibrium shape is crucial in defining what constitutes a planet. Hydrostatic equilibrium refers to the state where the gravitational forces pulling the material inward are balanced by the pressure forces pushing outward. This balance allows the planet to maintain a rounded shape, which is a key characteristic of celestial bodies classified as planets.

In essence, if a celestial body does not have enough mass, its gravitational pull will be insufficient to overcome structural forces and allow it to achieve this rounded, stable form. This characteristic distinguishes a planet from smaller bodies like asteroids or comets, which often have irregular shapes due to their insufficient mass.

The other options do not accurately reflect the defining characteristics of a planet. For example, while some planets may be gas giants, being made of gas alone does not qualify a body as a planet. Having multiple moons is not a criterion for being classified as a planet; many planets have moons, but not all that do are considered planets. Similarly, not all planets need to rotate around the Sun in a conventional sense, as there are celestial bodies like rogue planets that wander through space without a stable orbit around a star.