Which special instruction relates to the command to commence firing?

The instruction "At my command" is crucial because it designates the specific moment when the crew is authorized to begin the firing sequence. This command is intended to ensure that all personnel involved are ready and synchronized before engaging in the operation, emphasizing the importance of coordination and safety in artillery operations. Issuing this command allows the commander to manage the timing of fire effectively, which can be critical in a tactical situation, ensuring that the crew responds at the right moment to either exploit a tactical advantage or adhere to safety protocols.

The other choices, while relevant to different aspects of artillery operations, do not directly pertain to the command for firing initiation. For instance, "Do not load" serves as a safety instruction, indicating that the howitzer should remain unloaded until the right moment, which is separate from the timing of the firing command. "Sweep and zone" typically pertains to the procedures for checking the area around the firing position, while "High angle" refers to a specific firing technique rather than a command structure. Hence, the instruction that directly relates to when to commence firing is indeed "At my command."