Assertion (A) : In Indian and China , the productivity per unit is very low .
Reason (R) : The 70% of world livestock population is in Indian and China but their contribution to the world farm product is less than 25% only.

Options

A. Both A and R are correct and R is the correct explanation of A
B. Both A and R are correct but R is not the correct explanation of A
C. A is true but R is false
D. A is false and R is true

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**: The assertion states that "In India and China, the productivity per unit is very low." This implies that even though these countries have a significant number of livestock, the output (like milk, meat, wool, etc.) per animal is not high.

2. **Understanding the Reason (R)**: The reason provided is that "70% of the world livestock population is in India and China, but their contribution to the world farm product is less than 25% only." This indicates that despite having a large livestock population, the overall contribution to global farm production is minimal.

3. **Analyzing the Relationship**: The reason explains why the assertion is true. If India and China have a large proportion of the world's livestock but contribute less to farm production, it suggests inefficiencies in productivity. This could be due to traditional farming methods, lack of technological advancement, or other factors affecting livestock management.

4. **Conclusion**: Since both the assertion and the reason are true, and the reason effectively explains the assertion, we conclude that both A and R are correct, and R is the correct explanation of A.

5. **Final Answer**: Therefore, the answer is that both A and R are correct, and R is the correct explanation of A.