The correct answer is: a) Gandhara.
Gandhara was a region in the northwest of the Indian subcontinent, corresponding to modern-day eastern Afghanistan and northwestern Pakistan. It was one of the 16 Mahajanapadas, or major kingdoms, of ancient India. The capital of Gandhara was Taxila, which was an important center of trade and learning.
Magadha was a kingdom in the eastern Indian subcontinent, corresponding to modern-day Bihar and Jharkhand. It was one of the 16 Mahajanapadas, and it eventually became the most powerful kingdom in India. The capital of Magadha was Pataliputra, which was also an important center of trade and learning.
Vanga was a kingdom in the eastern Indian subcontinent, corresponding to modern-day Bangladesh and West Bengal. It was one of the 16 Mahajanapadas, and it was known for its fertile land and its production of rice. The capital of Vanga was Tamralipti, which was an important port city.
Avanti was a kingdom in the central Indian subcontinent, corresponding to modern-day Madhya Pradesh. It was one of the 16 Mahajanapadas, and it was known for its military strength and its production of horses. The capital of Avanti was Ujjain, which was an important religious and cultural center.
In conclusion, the correct answer is: a) Gandhara.