**Core Concept**
The question tests the student's knowledge of glucocorticoids, specifically their duration of action. Glucocorticoids are a class of **corticosteroids** that play a crucial role in regulating the body's response to stress, immune response, and inflammation.

**Why the Correct Answer is Right**
The longest acting glucocorticoids are typically used for their potent anti-inflammatory effects. Among the options, **Dexamethasone** is known for its long duration of action, making it suitable for various clinical applications. Its potency and long half-life allow for less frequent dosing.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because it is not specified, and without the option text, it's impossible to determine its accuracy.
**Option B:** and **Option C:** are also not provided, making it challenging to explain their incorrectness directly.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that **dexamethasone** has a long biological half-life, which makes it useful in clinical scenarios where prolonged glucocorticoid activity is desired. This is particularly relevant in managing conditions like **cerebral edema** or in **chemotherapy** protocols.

**Correct Answer:** D. Dexamethasone