Option b is the correct answer. Assertion (A) is true: Balban achieved a stable government by consolidating authority in his hands. He curbed the power of the nobles, abolished the Chahalgani system, and introduced the "Sijda" and "Paibos" customs to enforce his supremacy. This centralized control aimed to prevent internal rebellions and establish a strong Sultanate. Reason (R) is true: Balban was deeply concerned about the threat of Mongol invasions from the North-West frontier. He adopted a policy of "blood and iron," fortifying defenses, strengthening the western borders, and deploying troops to counter Mongol incursions. His focus on military preparedness aimed to safeguard his kingdom from potential Mongol invasions. (R) is not the correct explanation of (A): While it's true that Balban was concerned about the Mongol threat and took steps to strengthen the western frontier, this reason alone doesn't directly explain why he centralized authority in his hands. Balban's centralization of authority was a broader policy aimed at maintaining control over the nobles and maintaining a strong and stable government.