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