The great Asian river Mekong does not run through :

The Mekong River flows through six countries in mainland Southeast Asia — originating on the Tibetan Plateau in China and then passing through Myanmar, Laos, Thailand, Cambodia and finally Vietnam where it drains into the South China Sea. Authoritative basin summaries list China, Myanmar, Laos, Thailand, Cambodia and Vietnam as the river’s countries and describe the river’s Upper (including China) and Lower (Thailand, Lao PDR, Cambodia, Vietnam) basins [2]. Malaysia is not among the Mekong countries or within the Mekong basin, so the river does not run through Malaysia. Therefore option 2 (Malaysia) is the correct choice.

Sources

1. [1] https://documents1.worldbank.org/curated/en/964141608033220674/pdf/Mekong-Vision-3-0.pdf
2. [2] https://www.mrcmekong.org/geography/