The ancient city of Taxila, a UNESCO World Heritage site in Pakistan, is particularly renowned for its significant role as a prominent center of:

The correct answer to the question "The ancient city of Taxila, a UNESCO World Heritage site in Pakistan, is particularly renowned for its significant role as a prominent center of:" is Option B: Gandhara Civilization and Buddhist learning.

The other options — A) Harappan Civilization and urban planning, C) Mughal architecture and horticulture, D) Sassanian empire's administrative hub — are incorrect for this Pakistan Affairs question. As explained above: Taxila was a major center of the Gandhara civilization, famous for its unique Greco-Buddhist art and as a hub for Buddhist learning and monasteries from the 6th century BCE to the 5th century CE.