Answer:
Option C.
Step-by-step explanation:
The given function is

We need to find the domain and range of the given step function.
is least integer function. The output of this function is all integers and it is defined for all values of x.
The given function is 3 times of least integer function. So, the given function f(x) is defined for all real number and outputs of the function are all integers that are multiples of 3.
Domain of f(x) = All real numbers = (-∞,∞)
Range of f(x) = All integers that are multiples of 3.
Therefore, the correct option is C.