To solve the given question regarding the properties of varactor diodes, let's first understand what a varactor diode is and its key properties.

Understanding Varactor Diodes:

Varactor diodes, also known as varicap diodes, are a type of semiconductor diode whose junction capacitance varies with the applied reverse bias voltage. This property is used in electronic tuning circuits, such as voltage-controlled oscillators, frequency modulators, and frequency multipliers.

Analysis of the Given Statements:

1. Junction capacitance varies with bias voltage (Statement A): This is true. The capacitance of a varactor diode is dependent on the reverse bias voltage applied across its junction. As the voltage increases, the capacitance decreases.
2. Junction capacitance varies with time (Statement B): This statement is incorrect. The capacitance primarily varies with the bias voltage and not directly with time.
3. Non-linearity of varactor diodes makes them suitable for frequency multipliers (Statement C): This is true. The non-linear nature of the varactor diode's capacitance-voltage relationship allows it to be used for frequency multiplication of the input signal.
4. Linearity of varactor diode makes them suitable for frequency multipliers (Statement D): This is incorrect. Varactor diodes are inherently non-linear, which is why they are used in applications like frequency multipliers.

Conclusion:

The correct answer is A and C only.

Therefore, the correct properties of varactor diodes related to the context given are:

• The junction capacitance varies with bias voltage.
• The non-linearity makes them suitable for frequency multipliers.

This analysis correctly excludes options B and D, affirming that junction capacitance does not vary directly with time and that linearity does not make varactor diodes suitable for frequency multiplication.