Option a is the correct answer. Purana Quila, one of the ancient forts in Delhi, is credited to both the Mughal emperor Humayun and the Afghan King Sher Shah Suri. While Humayun laid its foundation, Sher Shah Suri continued its construction. The fort features three gateways and is encompassed by a broad moat connected to the river Yamuna. The northern gateway displays a unique blend of Islamic pointed arch and Hindu Chattris, showcasing the diverse architectural influences present in the fort's design.