Option a is the correct answer. Nalanda Vihar was destroyed by Bakhtiyar Khalji, a military general under Muhammad Ghori. In the late 12th and early 13th centuries, Khalji led military campaigns that resulted in the conquest of parts of eastern India, including Bihar. His invasions led to significant damage to Buddhist establishments such as Odantapuri and Vikramashila, and the destruction of Nalanda University. His rapid advances and conquest of Bengal ultimately resulted in the downfall of the Sena dynasty's rule in the region.