What characterizes the function f(x)?

The defining characteristic of a function is that it assigns exactly one output value for every input value. Thus, if you have a relationship where for each input (x-value) there is a unique output (y-value), that relationship meets the criteria of being a function. This consistency ensures that no matter how many times you input the same value, the output will always be the same.

Other options do not align with the definition of a function. For instance, if an equation has multiple outputs for a single input, it would not be classified as a function. Similarly, having at least one variable does not necessarily indicate a functional relationship, as many equations can contain multiple variables without being a function. An equation with no output variables cannot define a function either, as functions inherently require an evaluation of outputs based on given inputs.