Of which country is Vientiane the capital?

Option D

Laos, landlocked country of northeast-central mainland Southeast Asia. It consists of an irregularly round portion in the north that narrows into a peninsula-like region stretching to the southeast. Overall, the country extends about 650 miles (1,050 km) from northwest to southeast. The capital is Vientiane, located on the Mekong River in the northern portion of the country.