The correct answer is C) rotor blades and landing gear. An obstruction on a helodeck (helicopter landing deck) refers to any object that could potentially interfere with or damage the helicopter's rotor blades or landing gear during takeoff and landing operations. The safe clearance of the rotor blades and proper touchdown of the landing gear are critical factors for the safe operation of the helicopter, so obstructions that could impact these components are considered hazards that must be identified and mitigated. The other answer choices are incorrect because they do not directly relate to the primary safety concerns for helicopter operations on a helodeck. Pilot visibility (A) and the loading/unloading of cargo or passengers (B, D) are important considerations, but they are not the focus of the term "obstruction" in this context.