On connecting a device X in a series circuit with a battery and a resistor, a current passes through the circuit. On reversing the polarity of the battery, the current in the circuit drops to almost zero. The device X may be a

The correct option is p-n junction diode.

Let us understand the behavior of the circuit and the device X step-by-step:
1. Understanding the behavior of a p-n junction diode:
A p-n junction diode is a semiconductor device that allows current to flow easily in one direction but blocks it in the opposite direction. This property is known as unidirectional conductivity.

2. Forward Bias vs. Reverse Bias:
When the p-side (anode) of the diode is connected to the positive terminal of the battery and the n-side (cathode) is connected to the negative terminal, the diode is said to be forward-biased. In this state, the depletion region narrows, and a significant current flows through the circuit.
When the polarity of the battery is reversed, the p-side is connected to the negative terminal and the n-side is connected to the positive terminal. The diode is now reverse-biased. In this state, the depletion region widens, offering extremely high resistance, and the current drops to almost zero (except for a negligible leakage current due to minority charge carriers).

3. Analyzing other options:
- p-type and n-type semiconductors: These are simple doped semiconductor materials behaving like resistors. Reversing the battery polarity will change the direction of the current, but its magnitude will remain the same.
- Capacitor: In a DC circuit with a battery and a resistor, a capacitor blocks current completely once it is fully charged, regardless of polarity.

Thus, the device X must be a p-n junction diode because it conducts current in one direction (forward bias) and blocks it when polarity is reversed (reverse bias).