Option a is the correct answer. Qutbuddin Aybak, after succeeding Muhammad Ghori, chose Lahore as his capital following his victory over Taj al-Din Yildiz. This decision was driven by strategic considerations, as Lahore held significance in maintaining control over Ghurid territories in northwestern India. After initially venturing towards Ghazni, Aybak eventually established Lahore as his capital due to its favorable geopolitical position and its potential to consolidate his rule over the region.