What is the meaning of a tributary?

Correct answer: A river that joins a main river

The direct answer is A: a tributary is a river that joins a main river. A river system usually has one larger main river and many smaller streams or rivers. These smaller rivers flow into the main river and add their water to it; therefore, the exact reasoning is that a tributary is identified by where it flows and not by its size, colour, source, or appearance. Option A is correct because it describes this joining relationship. Option B is wrong because a sea wave is a movement of seawater, not a river joining another river. Option C is wrong because a dry channel may carry no water, but it is not necessarily a tributary. Option D is wrong because a wall of ice is not a river; it may describe an ice formation. A useful exam cue is: tributary means a smaller river that feeds a larger river. Do not confuse it with a distributary, which flows away from the main river, often near a delta.