The domain of a function is
a. the set of all points on the function
b. the set of all first elements of the function
c. the set of all second elements of the function

B. The set of all first elements of the function.

Step-by-step explanation:

We have been given a incomplete sentence. We are asked to complete our given sentence.

We know that the domain of a function is all real values of independent variable for which a function is defined and gives exactly one output.

We know that independent variable is x, which is first element of a point, therefore, the domain of a function is the set of all first elements of the function.