Which of the following is a stone-hurling siege engine?

The onager is a type of ancient siege engine specifically designed for hurling large stones and projectiles during warfare. Its mechanism typically involves a torsion system, where the energy stored in twisted sinew or ropes is released to propel stones toward enemy fortifications. This capability makes the onager an effective device for breaching walls or causing chaos in besieged cities. The term "onager" is derived from the Latin word for "wild ass," due to the engine's ability to launch projectiles with unexpected force, much like the animal's kick.

In contrast, the ballista is a large crossbow-like weapon primarily used to shoot bolts or smaller projectiles, while the scorpio, a smaller version of the ballista, is typically used for the same purpose. The aries, known as a battering ram, is designed to break through gates and walls rather than hurl stones. Thus, the distinct characteristic of the onager as a stone-hurling siege engine makes it the correct answer.