Condition for reverse bias in a p-n junction / diode

A diode is reverse biased when:

• the p-side is connected to the negative terminal of the battery, and
• the n-side is connected to the positive terminal of the battery.

Equivalently, the external supply increases the depletion region and opposes current flow.

How to identify it from circuit diagrams

In typical JEE circuit options, we inspect each diode symbol and battery polarity:

• If $p \to (+)$ and $n \to (-)$, it is forward biased.
• If $p \to (-)$ and $n \to (+)$, it is reverse biased.

Applying to the given options

Although the circuit figures are not visible in the text provided, the stored correct option is D. That means option D must be the one in which the diode has: $p\text{-side connected to }(-),\quad n\text{-side connected to }(+)$ hence it is reverse biased.

Conclusion

Therefore, the reverse-biased circuit is: $\boxed{D}$