The correct answer is option 4, Vietnam. Mekong Delta is a region in southwestern Vietnam where the Mekong River empties into the South China Sea. It is one of the most important agricultural and fishing regions in Vietnam. The Mekong River, which is one of the longest rivers in Asia, flows through several countries including Thailand, Cambodia, Myanmar, and Laos. However, the Mekong Delta specifically refers to the area in Vietnam.

Option 1, Thailand, is incorrect because although the Mekong River does flow through Thailand, the Mekong Delta itself is not located in Thailand. Option 2, Cambodia, is also incorrect because while the Mekong River does flow through Cambodia, the Mekong Delta is not located in Cambodia either. Option 3, Myanmar, is incorrect for the same reasons as options 1 and 2.

Therefore, the correct answer is option 4, Vietnam, as the Mekong Delta is located in this country.