In a clamper circuit, the primary purpose is to shift the DC level of the input signal without altering the shape of the signal. This is achieved by adding a DC bias to the input signal. The necessary components in a clamper circuit include a diode, a capacitor, and a resistor.

- Diode: The diode is an essential component in a clamper circuit as it allows the flow of current in one direction while blocking it in the opposite direction. It helps in shifting the DC level of the input signal by allowing the capacitor to charge or discharge through it.

- Capacitor: The capacitor is another necessary component in a clamper circuit. It is connected in parallel to the diode and is charged or discharged depending on the input signal. The capacitor stores the charge and releases it during the negative half-cycle of the input signal, thereby shifting the DC level of the signal.

- Resistor: The resistor is also an important component in a clamper circuit. It is connected in series with the diode and capacitor. The resistor limits the current flowing through the circuit and helps in stabilizing the voltage across the capacitor.

- Independent DC Supply: The independent DC supply is not a necessary component in a clamper circuit. It is not required because the purpose of a clamper circuit is to shift the DC level of the input signal using the existing components. The diode, capacitor, and resistor work together to achieve this without the need for an additional DC supply.

Therefore, the correct answer is option 'D' - Independent DC Supply.