The correct answer is (a) Pakistan. Gwadar port is a deep-water port located in the Balochistan province of Pakistan. It is situated on the Arabian Sea, at the mouth of the Persian Gulf. The port is strategically important, as it provides a gateway to Central Asia and the Middle East. It is also a major economic hub, and is expected to play a significant role in the development of the China-Pakistan Economic Corridor (CPEC).
Option (b), Iran, is incorrect because Gwadar port is not located in Iran. Iran is a country located in the Middle East, bordering Pakistan to the east.
Option (c), India, is incorrect because Gwadar port is not located in India. India is a country located in South Asia, bordering Pakistan to the west.
Option (d), Sri Lanka, is incorrect because Gwadar port is not located in Sri Lanka. Sri Lanka is an island country located in the Indian Ocean, off the coast of India.