Assertion: Vasopressin is also called anti-diuretic hormone (ADH)
Vasopressin acts mainly at the kidney and stimulates resorption of water and electrolytes by the distal tubules and thereby reduces the loss of water through urine (diuresis).

Options

A. If both assertion and reason are true and the reason is the correct explanation of the assertion.
B. If both assertion and reason are true but reason is not the correct explanation of the assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false

Explanation

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that vasopressin is also known as anti-diuretic hormone (ADH). This is correct because vasopressin is indeed the hormone that regulates water balance in the body and is commonly referred to as ADH.

2. **Understanding the Reason**: The reason explains that vasopressin acts mainly at the kidneys, stimulating the resorption of water and electrolytes in the distal tubules. This action reduces the loss of water through urine, which is known as diuresis. This statement is also accurate.

3. **Source of Vasopressin**: Vasopressin is produced in the hypothalamus but is secreted from the posterior pituitary gland. This is important to note as it clarifies the origin of the hormone.

4. **Mechanism of Action**: Vasopressin targets the kidneys, specifically the nephrons, which are the functional units of the kidney. It enhances the reabsorption of water in the distal tubules and collecting ducts, which decreases urine volume and conserves water in the body.

5. **Conclusion**: Both the assertion and the reason are correct. The assertion correctly identifies vasopressin as ADH, and the reason accurately describes its function in the kidneys to reduce water loss through urine.

6. **Final Answer**: Therefore, the correct answer is that both the assertion and reason are true, and the reason correctly explains the assertion.