A boom is weakest when it is in which position?

The correct choice highlights that a boom is weakest when it is fully extended and at the lowest possible angle. This position places the maximum load on the boom due to the entire length of the boom being used to support a weight with minimal angle support. In this scenario, the leverage is greatest because a low angle means that the effective loading creates a significant moment, making the boom susceptible to bending or buckling.

This condition of full extension combined with a low angle reduces the boom's structural integrity against lateral forces and can lead to failure or excessive flex if the load exceeds the boom's capacity. This is particularly important in rigging where understanding the load dynamics can significantly influence safety and operational success.

In contrast, the other positions involve configurations that distribute forces in a manner that provides greater structural support and stability. Half retracted positions, low angles in fully retracted positions, or medium angles with partial extension all tend to lower the amount of force exerted on the boom in a way that enhances its strength and resilience during operations.