Final answer:
The Marathas posed the most challenge to the Mughal Empire, with significant military successes under leaders like Shivaji. Although they extended their control significantly, their expansion was eventually challenged by the Afghan king, Ahmad Shah Durrani.
Step-by-step explanation:
The group that challenged the Mughal Empire the most was the Marathas. Shivaji, one of their most notable leaders, regularly defeated Mughal commanders and even managed to withstand a siege for over two years. The Marathas' military successes and control over territories, such as the port of Surat, contributed significantly to their challenge against the Mughal Empire. Additionally, the Marathas continued to grow in power even as the Mughal Empire weakened, ultimately gaining control of a sizeable portion of India.
However, their expansion came to a pause when Ahmad Shah Durrani, the Afghan king, marched against both Mughals and Marathas. These internal struggles also coincided with the gradual increase in European colonization of India.