Who is responsible for describing buoyant force?

The correct choice is Archimedes, who is credited with defining the principle of buoyant force, often referred to as Archimedes’ Principle. This principle states that any object submerged in a fluid experiences an upward force equal to the weight of the fluid that the object displaces. Archimedes' work in ancient Greek science laid the foundation for understanding buoyancy and fluid dynamics, which remains critical in various fields such as engineering and physics today.

While other figures mentioned, like Achilles, Pilates, and Einstein, are well-known in their respective domains—Achilles in mythology, Pilates in fitness, and Einstein in theoretical physics—they do not have contributions directly related to the concept of buoyant force. Thus, it is Archimedes who is specifically recognized for this significant scientific achievement.