Answer:
The answer to that question isn’t clear, so it’s safe to choose it depending on your learning source.
The most common answer would be B - Nile. But it could be also C - Amazon. Details below.
Step-by-step explanation:
Mississippi River - 3 765.87 km - second longest in North America
Nile - 6 650 km - major north-flowing river in northeastern Africa
Amazon - 6 400 km or 7 025 km
„The length of the Amazon River is usually said to be "at least" 6,400 km (4,000 mi), but reported values lie anywhere between 6,275–7,025 km (3,899–4,365 mi).”
Yangtze - 6 300 km - longest river in Asia, the third-lognest in the world