Functions, which are represented by different formulas on different subsets of its domain are called piecewise defined functions, so the correct answer is C. Definitions for the other concepts regarding to functions in the other options are as follows:

Constant function and identity function are the concepts related to types of functions. A function assigning each element from its domain a single element in its range is called a constant function.

Given A ? , a function defined on the set A and assigning every element of A to itself is called the identity function.

Given a function  if the image is equal to the range, i.e. f (A)=B the function f  is called surjective (onto).

If a function which is both one-to-one and onto is called a bijection.