The correct answer is A) Electric current. A solenoid valve is a type of valve that is actuated by an electromagnetic coil. When an electric current passes through the coil, it creates a magnetic field that pulls the valve's plunger or armature, which opens or closes the valve. This makes electric current the power source that actuates a solenoid valve. The other answer choices are incorrect because a solenoid valve is not actuated by air pressure (B), hydraulic pressure (C), or mechanical force (D). Those types of valves are operated by different mechanisms, not a solenoid.