The correct answer is  Cambodia.

• Laos:
  • ​It is a landlocked nation in Southeast Asia, northeast of Thailand, west of Vietnam.
  • It is surrounded by Myanmar (Burma), Cambodia, the People's Republic of China, Thailand, and Vietnam.
• Serbia:
  • ​It is a landlocked country situated at the crossroads of Central and Southeast Europe in the southern Pannonian Plain and the central Balkans.
• Ethiopia:
  • ​It is a landlocked country on the Horn of Africa.
  • It shares borders with Eritrea to the north, Djibouti to the northeast, Somalia to the east, Kenya to the south, South Sudan to the west, and Sudan to the northwest.
• Cambodia:
  • It is a country in mainland Southeast Asia, bordering Thailand, Laos, Vietnam, the Gulf of Thailand.
  • It is not a landlocked country, it shares a border with the Gulf of Thailand.

• The largest landlocked country is Kazakhstan in Central Asia with a land area of over 2.7 million kilometers. 
• The smallest landlocked country is the city-state of Vatican City with an area of just 44 hectares (109 acres). 

List of Continent-wise Landlocked Countries:

• Asia: Mongolia, Laos, Nepal, Bhutan Afghanistan, Kyrgyzstan, Tajikistan, Uzbekistan.
• Europe: Belarus, Moldova, Macedonia, Serbia, Kosovo, Hungary, Austria, check, Slovakia, Luxembourg, Switzerland.
• Africa: Mali, Burkina Faso, Niger, chat, Central African Republic, South Sudan, Ethiopia, Uganda, Zambia, Botswana, Zimbabwe, Malawi, Lesotho, Swaziland.
• South America: Bolivia, Paraguay