Which of the following is a property of functions?

Functions are defined by the relationship between their inputs and outputs, where each input corresponds to exactly one output. This characteristic is known as the "unique output" property. In mathematical terms, for a relation to be classified as a function, it must satisfy the criterion that no input from the domain is mapped to more than one output in the range.

This ensures predictability and consistency in how inputs are transformed. For instance, if you consider a function f(x), for every value of x in the domain, there's one specific value f(x) in the range. This property is fundamental to understanding how functions operate and how they can be analyzed across various contexts, such as in algebra or calculus.

The other choices contain inaccuracies regarding the nature of functions. Functions can exhibit various characteristics, including linearity, but they are not restricted solely to linear relationships. Additionally, there are essential mathematical constraints often placed on functions, meaning they may have domain and range restrictions, thereby invalidating the idea that they have no restrictions.