The point at which a stream empties into another body of water is the of the stream.

a. headwater b. drainage basin c. tributary d. mouth

The answer would be C, Tributary.  

Explanation:

A stream or River which flows into a larger river or stream is called as Tributary. This is the point where a stream empties itself into another body of water, such as ocean, lake, river or another stream. Tributary collects the water from the surroundings which is present in the form of drainage basins, surface water, ground water, etc and fall into the bigger stream of water. This larger stream is called the Mainstem.  

D. mouth

Explanation:

A stream is a body of running water that occupies a channel. Although it is most often found above ground, it can also be found underground. The place where a stream starts is known as source, while the place where the stream empties into another body of water is known as its mouth. The section located between the source and mouth is known as trunk.

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