## Core Concept
The question tests the understanding of mineralocorticoid activity among different steroids. Mineralocorticoids are a class of corticosteroids produced by the adrenal cortex that play a crucial role in regulating electrolyte and water balance in the body, primarily through their effects on the kidneys. The most well-known mineralocorticoid is aldosterone.

## Why the Correct Answer is Right
Corticosterone has relatively low mineralocorticoid activity compared to aldosterone. Aldosterone is the primary mineralocorticoid hormone in humans and has potent effects on sodium reabsorption and potassium excretion in the collecting ducts of the kidneys. Corticosterone, on the other hand, has some mineralocorticoid activity but significantly less than aldosterone. It is more known for its glucocorticoid effects.

## Why Each Wrong Option is Incorrect
* **Option A:** Aldosterone is the primary mineralocorticoid and has the highest mineralocorticoid activity, making it incorrect as the answer for the least mineralocorticoid activity.
* **Option B:** Deoxycorticosterone (DOC) has significant mineralocorticoid activity, although less than aldosterone. It is a potent mineralocorticoid and thus not the correct answer.
* **Option D:** Fludrocortisone has considerable mineralocorticoid activity and is used clinically for its combined glucocorticoid and mineralocorticoid effects, particularly in patients with adrenal insufficiency.

## Clinical Pearl / High-Yield Fact
A key point to remember is that among the given options, **cortisone (or Corticosterone in some contexts)** has the least mineralocorticoid activity, which makes it more favorable in certain clinical scenarios where glucocorticoid effects are desired with minimal mineralocorticoid activity.

**Correct Answer: C. Corticosterone**