Water and small solutes enter kidneys during :

Options

A. filtration
B. tubular secretion
C. tubular reabsorption
D. both (a) and (c)

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the process through which water and small solutes enter the kidneys.

2. **Identifying the Processes**: There are three main processes involved in kidney function:
- Filtration
- Tubular reabsorption
- Tubular secretion

3. **Analyzing Filtration**:
- Filtration is the first step in urine formation.
- It occurs in the glomerulus, where blood is filtered.
- During this process, a significant amount of blood (about 1100 to 1200 ml) is filtered, allowing water and small solutes to pass into the renal tubules.

4. **Analyzing Tubular Reabsorption**:
- Tubular reabsorption occurs after filtration.
- It involves the reabsorption of water and solutes back into the bloodstream from the renal tubules.
- While this process does involve water and solutes, it is not the initial entry into the kidneys.

5. **Analyzing Tubular Secretion**:
- Tubular secretion is the process of transferring substances from the blood into the renal tubules.
- This process does not involve the initial entry of water and small solutes into the kidneys.

6. **Conclusion**: The primary process through which water and small solutes enter the kidneys is filtration. Therefore, the correct answer is option **A**: filtration.

---