Pn Junction Diode

Step 1: Understanding the Concept:
The question describes a device that exhibits unidirectional current flow. It allows current to pass when connected to a battery in one polarity but blocks the current when the polarity is reversed. This is the defining characteristic of a rectifying device.

Step 2: Detailed Explanation:
Let's analyze the behavior of each device listed:
1. p-type semiconductor / 2. n-type semiconductor:
Both p-type and n-type semiconductors are simply resistive materials. They obey Ohm's law and conduct current regardless of the direction of the applied voltage. Reversing the battery polarity would simply reverse the direction of the current, but its magnitude would remain the same. They do not exhibit the described behavior.
3. p-n junction diode:
A p-n junction diode is specifically designed to allow current to flow in only one direction. When it is forward-biased (p-side connected to the positive terminal of the battery), it has very low resistance and a significant current flows. When it is reverse-biased (p-side connected to the negative terminal), it has very high resistance and the current drops to almost zero. This perfectly matches the description in the problem.
4. capacitor:
When a capacitor is connected to a DC battery, a transient charging current flows for a short time until the capacitor is fully charged. After that, the current stops (becomes zero). If the polarity is reversed, a transient discharging and then recharging current will flow in the opposite direction, eventually stopping again. It does not allow a steady current in one direction and block it in the other.

Step 3: Final Answer:
The device X, which allows current in one direction but blocks it in the reverse direction, is a p-n junction diode.