What action must you take to set the SECONDS TO ARM setting below 6 seconds on an M904 Fuze?

To set the SECONDS TO ARM setting below 6 seconds on the M904 Fuze, removing the stop screw from the fuze body is the correct action. The stop screw is a critical component that limits the minimum time setting for the fuse. By removing this screw, you are allowing for adjustments below the factory preset limit, which typically restricts the settings to a minimum of 6 seconds. This manipulation permits the user to configure the fuse according to specific operational requirements that necessitate a shorter arming time.

The other options do not provide a valid method for adjusting the SECONDS TO ARM setting below 6 seconds; turning the dial counterclockwise may only adjust the time within existing constraints, while replacing the timing mechanism or resetting the entire fuze unit would not allow for the specific adjustment needed without first removing the stop screw.