Answer:
The sound made by a duck.
Explanation:
There are two definitions that are the generally used for the word Quack:
"the characteristic harsh sound made by a duck"
"(of a duck) make a characteristic harsh sound"
Also, the Cambridge Dictionary defines "quack" as both, "The sound that a duck makes" and "a person who dishonestly pretends to have medical skills or knowledge", but the latter is less commonly used.
