Assertion: If electrons in an atom were stationary, then they would fall into the nucleus.
Reason: Electrostatic force of attraction acts between negatively charged electrons and positive nucleus.

Explanation

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

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states: "If electrons in an atom were stationary, then they would fall into the nucleus."
- This implies that if electrons are not in motion, the electrostatic attraction between the negatively charged electrons and the positively charged nucleus would cause them to move towards the nucleus.

2. **Understanding the Reason**:
- The reason states: "Electrostatic force of attraction acts between negatively charged electrons and positive nucleus."
- This is a true statement as per Coulomb's law, which states that opposite charges attract each other.

3. **Analyzing the Assertion**:
- If electrons were stationary, the only force acting on them would be the electrostatic force of attraction towards the nucleus.
- Since there is no opposing force to keep them in orbit (like centripetal force), they would indeed fall into the nucleus.

4. **Centripetal Force Requirement**:
- In a stable orbit, electrons are in motion, and the electrostatic force provides the necessary centripetal force to keep them in circular motion.
- If the electrons were to stop moving (become stationary), the centripetal force would no longer be required, and the electrostatic force would pull them directly towards the nucleus.

5. **Conclusion**:
- Both the assertion and the reason are true.
- The reason explains why the assertion is true, as it describes the electrostatic force that causes the electrons to fall into the nucleus when they are stationary.

### Final Answer:
- The assertion is true, the reason is true, and the reason explains the assertion. Therefore, the correct option is **1**.