What must be done when changing the arming time settings on the M905 tail fuze?

The correct choice is to depress the timing lock when changing the arming time settings on the M905 tail fuze. This step is crucial because the timing lock prevents accidental adjustment of the arming time during handling. By depressing the timing lock, you ensure that the mechanism is safely engaged and able to be modified without the risk of unintentional activation or change that could compromise the safety and effectiveness of the fuze.

In the context of handling a sensitive component like the M905 tail fuze, each action must be taken with precision to avoid any unintended consequences. Depressing the timing lock allows the operator to make necessary adjustments while maintaining safety protocols essential for managing munitions systems effectively.