Mekong river is world’s 12th longest river. Answer to your question is B. A is wrong because Mekong river begins in In Tibetan Plateau. Answer C is also wrong because it empties in South China Sea. D is also wrong because the length or the river is 4350Km(2703 mi). Answer B is right because Mekong river is trip point to Myanmar, Laos and Thailand. Trip point means when 3 country borders meet.