According to Newton's Third Law of Motion, the user would feel a reaction force of 121 N when using a fire extinguisher that sprays with a force of 121 N.
The question asked involves understanding the principles of Newton's Third Law of Motion, which states that for every action, there is an equal and opposite reaction. When a fire extinguisher sprays with a force of 121 N, the user would feel a reaction force of 121 N pushing back on them.
This force is a result of the high-speed ejection of contents from the extinguisher, which in turn pushes the extinguisher in the opposite direction with equal force.