Final answer:
The firefighting practice that gives the greatest benefit in reducing the need for extra manpower during salvage operations is to remove as much furniture or merchandise within the building as quickly as possible.
Step-by-step explanation:
The firefighting practice that gives the greatest benefit in reducing the need for extra manpower during salvage operations is option C: Remove as much furniture or merchandise within the building as quickly as possible. By removing furniture or merchandise, firefighters can create more space and reduce obstructions, making it easier to navigate and perform salvage operations in the building.