Why D? And why not the others?

Correct answer: D. The Amazon

The Amazon's drainage basin covers roughly 6 to 7 million square kilometers of South America, gathering water from the Andes to the Atlantic and making it by far the largest river basin on Earth, one that alone carries about a fifth of all the freshwater that runs off the planet's land into the ocean. The Congo basin is the genuine runner-up and Africa's own giant, but at around 3.7 million square kilometers it covers roughly half the Amazon's footprint. The Mississippi basin is enormous by North American standards, draining about 40% of the continental United States, yet it still totals only around 3.2 million square kilometers. The Nile is the trap for anyone reasoning from the river's famous length rather than its catchment: despite flowing further than any other single river, much of its course threads through narrow desert corridor rather than a broad watershed, leaving its basin at only a modest fraction of the Amazon's size.

Source: Encyclopaedia Britannica: 'Amazon Basin' (britannica.com/place/Amazon-Basin)