Blood clotting can be prevented in a test tube by adding a little
(a) Sodium oxalate
(b) Sodium chloride
(c) Sodium hydroxide
(d) Ammonium chloride

Options

A. Sodium oxalate
B. Sodium chloride
C. Sodium hydroxide
D. Ammonium chloride

Answer

To solve the question regarding which substance can prevent blood clotting in a test tube, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks which of the given options can act as an anticoagulant to prevent blood clotting in a test tube.

2. **Identify the Options**: The options provided are:
- (a) Sodium oxalate
- (b) Sodium chloride
- (c) Sodium hydroxide
- (d) Ammonium chloride

3. **Research Anticoagulants**: Anticoagulants are substances that prevent blood coagulation. One common anticoagulant is sodium oxalate, which is known to inhibit blood clotting.

4. **Analyze Each Option**:
- **Sodium Oxalate**: This compound acts as an anticoagulant by forming an insoluble complex with calcium ions, which are essential for the blood clotting process. Therefore, it prevents clotting.
- **Sodium Chloride**: This is common table salt and does not have anticoagulant properties.
- **Sodium Hydroxide**: This is a strong base and does not function as an anticoagulant.
- **Ammonium Chloride**: This compound does not have known anticoagulant properties.

5. **Conclusion**: Based on the analysis, the only option that prevents blood clotting is (a) Sodium oxalate.

### Final Answer:
**(a) Sodium oxalate**