Which of these countries does the Mekong river not flow through?

The Mekong river does not flow through Malaysia.

The Mekong is a trans-boundary river in Southeast Asia. It is the world's twelfth longest river and the seventh longest in Asia. Its estimated length is 4,350 km (2,703 mi), and it drains an area of 795,000 km2 (307,000 sq mi), discharging 475 km3 (114 cu mi) of water annually. From the Tibetan Plateau the river runs through China, Myanmar, Laos, Thailand, Cambodia, and Vietnam. The extreme seasonal variations in flow and the presence of rapids and waterfalls in the Mekong make navigation difficult. Even so, the river is a major trade route between western China and Southeast Asia.

Malaysia is a country in Southeast Asia. The federal constitutional monarchy consists of 13 states and three federal territories, separated by the South China Sea into two similarly sized regions, Peninsular Malaysia and East Malaysia (Malaysian Borneo). Peninsular Malaysia shares a land and maritime border with Thailand and maritime borders with Singapore, Vietnam, and Indonesia.