The correct answer is (a). Dhanyakataka was located in the Andhra region. It was a prominent Buddhist center under the Mahasanghikas. The Mahasanghikas were a Buddhist school that emerged after the Second Buddhist Council. They were one of the two major schools of Buddhism, the other being the Sthavira. The Mahasanghikas were more liberal than the Sthavira and they accepted a wider range of texts as canonical. Dhanyakataka was an important center for Mahasanghika Buddhism. It was the site of several important Buddhist councils and it was also a major pilgrimage destination.
The other options are incorrect. Gandhara was a region in northwestern India and Pakistan. It was an important center for Buddhism in the early centuries CE. Kalinga was a region in eastern India. It was an important center for Buddhism in the late centuries BCE and early centuries CE. Magadha was a region in central India. It was an important center for Buddhism in the early centuries BCE.