What function does a rainwater catchment tube serve on a life raft?

A rainwater catchment tube on a life raft serves the essential function of collecting rainwater. In survival situations at sea, access to fresh drinking water is crucial, and collecting rainwater provides a viable means to acquire this resource when other freshwater options are limited. The design of the tube allows the rainwater to flow into a designated storage area or container, ensuring that the collected water can be used for drinking.

While filtering drinking water is important in survival situations, the catchment tube's primary role is to facilitate the collection of rain, not purification. Storing food supplies is not relevant to the tube's function, as it is designed specifically for water collection. Cooling the raft could be beneficial in certain climates, but this is not a function served by the rainwater catchment tube. Thus, the effective collection of rainwater is critical for providing sustenance when stranded at sea, making this the correct answer.