A function is a relationship that each input (element of the domain) is paired with exactly one output (element in the range).
In function notation, f(x), the “x” is the input and the entire “f(x)” represents an output.
That means the last two options are wrong. They’re purposely switched up in what they say.
The correct answer is the second one, because that one is saying “each input will be paired with exactly one output.”