Which safety feature allows ground personnel to monitor the boom angle?

The correct choice is the boom angle indicator, as it is specifically designed to measure and display the angle of the boom in relation to the ground. This device is vital for ensuring that operators and ground personnel can monitor the height and reach of the crane safely. By providing real-time data on the boom's position, it helps prevent accidents related to overreaching or tipping, especially in complex lifting operations where visibility might be compromised.

The winch cable, sheave, and pulley are important components of a crane's lifting mechanism, but they do not provide information about the boom angle. The winch cable is used for lifting and lowering loads, while sheaves and pulleys assist in changing the direction of the force applied by the winch. None of these components offers a direct means of monitoring the boom's angle, which is essential for safe crane operation.