Option a is the correct answer. Shah Jahan's Balkh campaign was a military expedition that was launched in 1646. The objective of the campaign was to secure a friendly ruler in Balkh and Badakshan, which bordered Kabul. Balkh and Badakshan were important strategic regions that controlled the trade routes between Central Asia and India. They were also important for the defense of Kabul, which was the capital of the Mughal province of Afghanistan. Shah Jahan was concerned that the rulers of Balkh and Badakshan were becoming increasingly independent and that they could pose a threat to Mughal control of Kabul. He therefore launched a campaign to depose the rulers of Balkh and Badakshan and to install friendly rulers in their place. The Mughal campaign was successful and the rulers of Balkh and Badakshan were deposed. Shah Jahan then installed friendly rulers in their place. This secured the Mughal frontier in Central Asia and ensured the safety of Kabul.