The longest river in the world is the Amazon River, option 'C'. It stretches approximately 6,400 kilometers (4,000 miles) from its source in the Peruvian Andes to its mouth in the Atlantic Ocean in northeastern Brazil. The Amazon River is known for its vast size, volume of water, and the diverse ecosystem it supports.

Heading 1: The Amazon River's Length
The Amazon River is widely recognized as the longest river in the world. It surpasses other major rivers like the Nile and the Yangtze in terms of length. The Amazon's length is estimated to be around 6,400 kilometers (4,000 miles). This measurement takes into account the main stream of the river, excluding its tributaries.

Heading 2: Source and Course
The Amazon River originates from the Andes Mountains in Peru, near the city of Iquitos. Its main source is the Mantaro River, which arises from glacial meltwater. From its source, the river flows eastward through Peru, forming the border between Colombia and Brazil for a short distance, before entering Brazil entirely.

Heading 2: Tributaries
One of the remarkable characteristics of the Amazon River is its extensive network of tributaries. It has numerous tributaries, including major ones like the Madeira, Negro, and Japurá rivers. These tributaries significantly contribute to the overall length and volume of the Amazon River, making it the largest river system in the world by discharge.

Heading 2: Volume and Discharge
The Amazon River carries an immense volume of water, making it the largest river in terms of discharge. On average, it discharges around 209,000 cubic meters (53 million gallons) of water per second into the Atlantic Ocean. This is significantly more than the combined discharge of the next seven largest rivers in the world.

Heading 2: Importance and Ecosystem
The Amazon River and its surrounding rainforest are of great ecological and environmental significance. The river and its basin are home to an incredibly diverse range of plant and animal species, many of which are endemic to the region. The Amazon rainforest, often referred to as the "lungs of the Earth," plays a vital role in global climate regulation and is a major carbon sink.

In conclusion, the Amazon River is the longest river in the world, stretching over 6,400 kilometers (4,000 miles). Its extensive network of tributaries, vast volume of water, and the rich ecosystem it supports make it a truly remarkable natural feature.