How can a plumbing system ensure proper drainage?

A plumbing system ensures proper drainage primarily by maintaining proper venting. Venting plays a crucial role in allowing air to enter the plumbing system, which helps maintain atmospheric pressure while facilitating the movement of wastewater through the pipes. Proper venting prevents the formation of vacuum seals that can impede the flow of water, leading to slow drainage or potential blockages.

In addition, adequate venting allows sewer gases to escape, thus maintaining a safe and odor-free environment. Without proper venting, a drain may not function correctly, resulting in various issues like gurgling sounds or slow-draining fixtures.

The other options, while related to drainage system performance, do not address the essential need for venting as directly as the chosen answer. Increasing the slope can improve drainage to an extent, but if not balanced correctly with appropriate venting, it may still lead to drainage issues. Similarly, reducing pipe size may create resistance to flow, countering drainage efficiency, and installing more fixtures does not ensure drainage effectiveness without proper venting.