The correct answer is option 2, Laos. A land-locked country is one that does not have any coastline or access to the sea. Laos is a land-locked country located in Southeast Asia. It is bordered by several countries including China, Vietnam, Cambodia, Thailand, and Myanmar, but it does not have any access to the sea.

Option 1, Cambodia, is not a land-locked country as it has a coastline along the Gulf of Thailand. Option 3, Thailand, is also not a land-locked country as it has coastlines along the Andaman Sea and the Gulf of Thailand. Option 4, Vietnam, is also not a land-locked country as it has a long coastline along the South China Sea.

It is important to understand the geography of different countries and their access to the sea in order to determine if they are land-locked or not.