Option b is the correct answer. During Alauddin Khalji's reign after 1306 A.D., the boundary between the Delhi Sultanate and the Mongols was demarcated by the Ravi River. This geographical feature served as a natural barrier, helping to curb Mongol invasions and secure the northern frontiers of the empire. The Ravi River acted as a strategic defense line, contributing significantly to the stability, protection, and overall security of the realm during that period. Important Tips Mongol invasions and northern conquests, 1297-1306: During 1297-1306, Mongol invasions and conquests were marked by several key events in Alauddin Khalji's reign: Punjab Raid and Victory: In winter 1297, Mongols from the Chagatai Khanate raided Punjab, advancing to Kasur. Led by Ulugh Khan, Alauddin's forces defeated them in February 1298. About 20,000 Mongols were killed, and many more perished in Delhi after capture. Sindh Invasion: In 1298-99, another Mongol army invaded Sindh, occupying the fort of Sivistan. Alauddin's general Zafar Khan recaptured the fort, defeating the invaders. Gujarat Campaign: In early 1299, Alauddin sent Ulugh Khan and Nusrat Khan to invade Gujarat. They plundered towns, including Somnath's temple. The Delhi army captured Vaghela queen Kamala Devi and Malik Kafur. On the return from Gujarat, Mongol soldiers mutinied near Jalore over loot sharing. Alauddin's response included harsh punishments, reflecting the practice of punishing families for men's crimes. Battle of Kili: In 1299, Chagatai ruler Duwa sent Mongols led by Qutlugh Khwaja. In the Battle of Kili, Alauddin led Delhi forces, but Zafar Khan attacked without orders. Mongols retreated, inflicting heavy casualties.