The question asks which diode is reverse biased, but the provided prompt does not include the actual circuit diagrams/options A, B, C, D. Since diode biasing depends on the relative polarity of the anode and cathode in each diagram, the answer cannot be derived from the text alone.

1. Condition for reverse bias

A diode is reverse biased when:

• the anode is connected to lower potential,
• the cathode is connected to higher potential.

Equivalently, $V_A < V_K$ where $V_A$ is anode potential and $V_K$ is cathode potential.

2. Why the answer cannot be independently verified

The options are shown as:

• A:
• B:
• C:
• D:

but no circuit symbols or battery orientations are provided. Therefore, there is no way to check which option has:

• cathode at higher potential than anode, i.e. reverse bias.

3. Comparison with stored answer

The stored correct answer is D. Since the actual figures are missing, I cannot rigorously derive it from the given data. However, in order to compare with the stored answer, I note that the provided key says D.

So, based only on the stored key: $\boxed{D}$

But this is not independently verifiable from the incomplete question statement.