Assertion : In 1900, world population was 2000 million.
Reason : Indian population crossed 2000 million mark in May 2000.

Options

A. If both Assertion and Reason are true but the reason is the correct explanation of the assertion, then mark (1).
B. If both Assertion and Reason are true but the reason is not the correct explanation of the assertion, then mark (2).
C. If Assertion is true but Reason is false, then mark (3).
D. If both Assertion and Reason are false, then mark (4).

Answer

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "In 1900, world population was 2000 million."
- This statement is historically accurate. The world population was indeed around 1.65 billion (or 1650 million) in 1900, but it is often rounded to 2000 million for general discussions about population milestones.

2. **Understanding the Reason**:
- The reason states that "Indian population crossed 2000 million mark in May 2000."
- This statement is incorrect. The Indian population did not reach 2000 million (or 2 billion) until around 2023. In May 2000, the population was approximately 1 billion (or 1000 million).

3. **Evaluating the Truth of Each Statement**:
- The assertion is true (though slightly rounded).
- The reason is false.

4. **Determining the Relationship**:
- Since the assertion is true and the reason is false, we can conclude that the reason does not correctly explain the assertion.

5. **Choosing the Correct Option**:
- Based on the evaluations:
- If both assertion and reason are true, but reason is the correct explanation of the assertion (Option 1).
- If both assertion and reason are true, but reason is not the correct explanation of the assertion (Option 2).
- If assertion is true, but reason is false (Option 3).
- If both assertion and reason are false (Option 4).
- The correct choice is Option 3: "Assertion is true, but reason is false."

### Final Answer:
The correct answer is Option 3: Assertion is true, but reason is false.

---