The correct answer is C) R. The R knot, also known as the rolling hitch or Magnus hitch, is the appropriate knot to use when securing a bosun's chair to a gantline. This knot is designed to grip the gantline securely while allowing the chair to be easily adjusted or released as needed. The rolling hitch is a versatile knot that resists slipping and is commonly used for attaching lines to cylindrical objects like spars or gantlines. The other answer choices are not suitable for this application. Knots I and X are not as secure or adjustable as the rolling hitch, while the P knot, or prussik knot, is better suited for climbing applications rather than attaching a bosun's chair.