The correct answer is C) Some unusual handling characteristics. Compared to a fixed-pitch propeller, a controllable-pitch propeller can have some unusual handling characteristics that a vessel operator needs to be aware of. The ability to change the pitch of the propeller blades provides more control over the vessel's speed and thrust, but it can also introduce different responses and feel during maneuvering. This can require the operator to adjust their handling techniques compared to a more predictable fixed-pitch propeller. The other options, such as lack of directional control when backing (A), inefficiency at high shaft RPM (B), or slightly higher fuel consumption (D), are not inherent disadvantages of the controllable-pitch design itself.