Two firefighters are fighting a fire with identical water hoses and nozzles, except that one is holding the hose straight so that the water leaves the nozzle in the same direction it comes, while the other holds it backward so that the water makes a u-turn before being discharged. which firefighter will experience a greater reaction force?

The one that holds it backward will experience a greater reaction force.

Explanation:

Newtons third law of motion states that in every action there is equal and opposite reaction. The action force of making the water  leave the nozzle for the firefighter who holds the hose backwards is greater than that for the fire fighter holding it straight. The water requires a a greater momentum and force to make a u-turn before leaving the nozzle. Hence the reaction force for the hose held backwards is greater