What term describes the action of lying stretched out on the ground with one's face downward?

The term that describes the action of lying stretched out on the ground with one's face downward is "prostrate." This word captures the specific position of being lying flat on the ground, often suggesting a state of helplessness or submission. It is derived from the Latin root "prostratus," which means to lay flat or spread out.

In contrast, the other options pertain to different meanings: "revelry" refers to lively and noisy festivities, typically involving dancing and drinking; "acrid" describes a strong and unpleasant smell or taste; and "capacious" means having a lot of space or being able to hold much. These definitions highlight that they do not relate to the act of lying down in the manner specified in the question. Thus, "prostrate" is the most appropriate choice for the described action.