The answer is option 'A' which means that 20,000 troops of Gwalior went over to Tantia Tope and Rani Lakshmi Bai. This event took place during the Indian Rebellion of 1857.

Background
The Indian Rebellion of 1857, also known as the First War of Indian Independence, was a widespread uprising against British rule in India. The rebellion began in May 1857 and lasted for almost two years. The rebellion was sparked by a number of factors, including religious and cultural grievances, economic hardship, and resentment towards British rule.

Gwalior's Role
Gwalior was an important princely state in central India during the British Raj. The state was ruled by the Scindia dynasty, which had close ties with the British. However, during the rebellion, the Scindia ruler was initially hesitant to join the rebellion. This changed when Rani Lakshmi Bai, the queen of the nearby state of Jhansi, joined the rebellion.

The Rebellion
When Rani Lakshmi Bai joined the rebellion, many soldiers from Gwalior also joined her. This included around 20,000 troops, who went over to the side of Tantia Tope and Rani Lakshmi Bai. This was a significant boost for the rebel cause, as it added a large number of trained soldiers to their ranks.

The Aftermath
Despite the support of Gwalior's troops, the rebellion ultimately failed. The British were able to suppress the rebellion and reassert their control over India. However, the rebellion had a lasting impact on Indian history. It led to major reforms in the British administration of India and laid the groundwork for the Indian independence movement.

Conclusion
In conclusion, during the Indian Rebellion of 1857, 20,000 troops of Gwalior went over to Tantia Tope and Rani Lakshmi Bai. This was a significant moment in the rebellion, as it added a large number of trained soldiers to the rebel cause. While the rebellion ultimately failed, it had a lasting impact on Indian history.