Afunction of the form f(x) = mx + b, where m and b are real numbers, is called a function.

This is a linear function.  Note that the independent variable, x, has the first power.  Were the function g(x) = mx^2 + b, you'd have a quadratic function.

It's important that you be able to envision functions such as the constant function, linear functions, quadratic functions and cubing functions quickly when you encounter them.

The first one is correct