Explanation:

- Gandhara was one of the significant Mahajanapadas in ancient India.

- Option 1: Mathura - It was not the capital of Gandhara but rather an important city in other contexts, such as in the kingdom of the Shurasena.

- Option 2: Varanasi - Also known as Kashi, it was an independent Mahajanapada and not associated with Gandhara.

- Option 3: Taxila - This was indeed the capital of the Gandhara Mahajanapada, famous for its ancient university and prosperous culture. .

- Option 4: Hastinapur - Known from the epic Mahabharata, it was not linked with Gandhara.

Understanding these key historical contexts can help in appreciating the rich tapestry of India's cultural history.