The pairs correctly matched are Volga - Caspian Sea and Zambezi - Indian Ocean. The Volga River flows into the Caspian Sea, which is the largest enclosed inland body of water on Earth, representing an essential geographical feature in terms of drainage systems in Europe and Asia. The Zambezi River, known for its spectacular waterfalls and ecosystems, indeed flows into the Indian Ocean, playing a crucial role in the hydrology of southern Africa. The other pairs, Mekong (which flows into the South China Sea) and Thames (which flows into the North Sea), are incorrectly matched, highlighting the importance of understanding river systems and their corresponding outlets in geographic studies pertinent to UPSC preparation.