The correct answer is: C. Both S1 & S2 are true.
In a PMMC writing system, the magnitude of pen deflection is proportional to the current flowing through the coil. The coil is made of a thin wire that is wound around a soft iron core. When a current flows through the coil, it creates a magnetic field. The magnetic field interacts with the magnetic field of the permanent magnet, which causes the coil to rotate. The amount of rotation is proportional to the amount of current flowing through the coil. The pen is attached to the coil, so the rotation of the coil causes the pen to move. The amount of movement of the pen is also proportional to the amount of current flowing through the coil.
The voltage in the coil is not directly proportional to the magnitude of pen deflection. The voltage in the coil is proportional to the current flowing through the coil, but the current flowing through the coil is not the only factor that affects the magnitude of pen deflection. The resistance of the coil and the strength of the permanent magnet also affect the magnitude of pen deflection.