What causes natural draft to occur?

Natural draft occurs due to the difference in density between warm and cool air. When air is heated, it expands and becomes lighter, causing it to rise. Cool air, being denser, will then move in to take its place. This movement creates a natural circulation of air, leading to a draft without the need for mechanical assistance.

In the context of stationary engineering, natural draft is particularly important for systems such as boilers and chimneys, where the movement of air and gases is crucial for efficient operation. Option A accurately describes this phenomenon by highlighting that the weight of warm air is less than that of cool air, driving the natural flow.

The other options refer to methods of creating draft through mechanical means or enhancing circulation, which do not fit the definition of natural draft.