Which device is primarily used to prevent two-blocking in crane operations?

The device primarily used to prevent two-blocking in crane operations is the anti two-blocking device. This safety mechanism is designed to detect when the hook or load block has come too close to the boom tip. When this condition is detected, the device activates to prevent further movement, which helps to avoid the dangerous situation known as two-blocking. Two-blocking can cause significant damage to the crane equipment and poses serious safety risks by potentially leading to the crane's collapse or the load falling.

Other options such as the boom angle indicator, radius indicator, and hoist drum indicator serve different purposes in crane operation. The boom angle indicator provides information on the angle of the boom, which is useful for operators to know the position of the crane but does not specifically prevent two-blocking. The radius indicator measures the distance from the center of rotation to the hook, assisting operators in understanding the crane's capacity based on its configuration. The hoist drum indicator gives information about the amount of wire rope that has been wound onto the drum but does not actively prevent two-blocking. Therefore, the anti two-blocking device is the specialized equipment focused directly on preventing this specific hazard.