Which of the following is NOT a type of propeller?

A propeller is an essential component in marine engineering, designed to convert rotational motion into thrust to propel a vessel through water.

The term "variable flow" is not recognized as a standard type of propeller. In contrast, solid propellers, controllable pitch propellers, and built-up propellers are all established types, each with distinct characteristics and uses. Solid propellers feature a single, solid piece design, controllable pitch propellers can adjust the angle of their blades to optimize performance for varying conditions, and built-up propellers allow for some modular construction, making repairs and modifications more manageable.

Understanding the various types of propellers helps clarify why "variable flow" is not applicable in this context, as it does not pertain to propeller design or function in any recognized way.