If a weapon cannot be rendered safe, a foreign object should be placed between the _______ and the _________.

The correct choice of placing a foreign object between the hammer and the frame is based on the function and mechanics of firearms. In a firearm, the hammer is a crucial component that strikes the firing pin to initiate a shot. If a weapon cannot be rendered safe, placing a foreign object between the hammer and the frame prevents the hammer from moving and striking the firing pin. This action significantly reduces the risk of an accidental discharge.

The other options present different parts of the firearm that do not serve the same safety function in this context. For example, placing an object between the barrel and the slide does not prevent the firing mechanism from operating. Similarly, putting a foreign object between the trigger and guard or between the receiver and stock does not effectively prevent the weapon from discharging. Therefore, the placement of a foreign object specifically between the hammer and the frame is a recognized safety measure to secure the firearm when it cannot be rendered safe in traditional manners.