The correct answer is C) When the boom is raised to a maximum permissible height or lowered to a minimum permissible height, the luffing pump shall be stroked to zero and the luffing winch brake set. This is the correct answer because the luffing motion of the crane must be stopped and secured when the boom is at its maximum or minimum permissible height. Placing the luffing pump at zero stroke and setting the luffing winch brake ensures that the boom cannot inadvertently move and potentially cause damage or injury. The other options are incorrect because they do not properly describe the required actions to safely limit the luffing motion of the crane at its height extremes. Releasing the luffing winch brake or keeping the luffing pump on stroke could allow uncontrolled movement of the boom, which would be unsafe.