**Core Concept**
The question is testing the relative potencies of different steroids, specifically their glucocorticoid activity. Glucocorticoids are a class of steroid hormones that play a crucial role in regulating metabolism, immune response, and inflammation.

**Why the Correct Answer is Right**
The correct answer is **Dexamethasone**. Dexamethasone is a synthetic glucocorticoid that is approximately 25-30 times more potent than hydrocortisone (cortisol). Its high potency is due to its ability to bind to glucocorticoid receptors with high affinity, which allows it to exert significant anti-inflammatory and immunosuppressive effects at lower doses.

**Why Each Wrong Option is Incorrect**
**Option A:** Hydrocortisone (cortisol) is less potent than dexamethasone due to its lower affinity for glucocorticoid receptors.
**Option B:** Prednisolone is an intermediate-acting glucocorticoid that is less potent than dexamethasone, although it is more potent than hydrocortisone.
**Option C:** Methylprednisolone is a potent glucocorticoid, but it is not as potent as dexamethasone.

**Clinical Pearl / High-Yield Fact**
Dexamethasone is often used in critical care settings for its potent anti-inflammatory effects, but it can also have significant side effects, including immunosuppression and adrenal suppression.

**Correct Answer:** D. Dexamethasone