What architectural feature in Pompeii was often used to collect rainwater?

The impluvium is the architectural feature in Pompeii specifically designed to collect rainwater. It is a shallow basin typically located in the center of the atrium, where water from the open roof (compluvium) would flow down into it during rain. This not only served a practical function by providing a water source for the household, but it also played a role in enhancing the aesthetic appeal of the atrium, which was often a central area of a Roman house.

In contrast, other features like the fauces, atrium, and vestibulum have different purposes. The fauces is the narrow passage that leads from the entrance of the house into the atrium, while the atrium is the main reception area of a Roman house, which may contain the impluvium but is not solely for collecting water. The vestibulum serves as a forecourt or entrance that connects the exterior of the house to the interior spaces, but it does not have a water collection function. Thus, the correct answer highlights the specific design and purpose of the impluvium in Roman architectural practices.