What mathematical concept defines what you put into a function?

The correct answer reflects the concept of the domain in a function. The domain is defined as the set of all possible input values that can be used in the function. In other words, when you refer to what you can put into a function, you're talking about the values that the independent variable can take on, which make up the domain.

For example, in a function where you have a relationship between x and y, the domain includes all the x-values (inputs) that are valid based on the function's definition. Understanding the domain is essential to gather or compute the outputs (dependent variable values) effectively, ensuring that inputs lead to meaningful results within the scope of the function.

The other options represent different concepts: the input refers to the values you enter into the function, the output refers to the results or values produced by the function, and the range is the set of possible output values. While these terms are related to the function's operation, it is the domain that specifically defines the permissible inputs.