Answer:
D. 13.8 N
Step-by-step explanation:
This problem can be solved by applying Pascal's principle, which states that the pressure is transmitted equally through all of the parts of the fluid inside the hydraulic system. So, the pressure on the two ends of the system is equal:

where:
 is the force on the first piston
 is the force on the first piston
 is the surface area of the first piston
 is the surface area of the first piston
 is the force to be applied on the activator end
 is the force to be applied on the activator end
 is the surface area of the activator end
 is the surface area of the activator end
Re-arranging the equation, we get:
