The correct answer is D. A constant-tension mooring winch is designed to automatically render and recover mooring line using the prime mover when the mooring line tension varies outside of narrow preset adjustable tension limits. This ensures the mooring line maintains a relatively constant tension, preventing excessive strain on the line or mooring hardware. The other options are incorrect because they do not accurately describe the functionality of a constant-tension mooring winch. Options A and C describe the winch only rendering the line, not both rendering and recovering, which is a key feature of a constant-tension winch. Option B describes the winch only recovering the line, not both rendering and recovering.