Answer: R
Step-by-step explanation: The domain of a function is the set of all x-values of the function.
The x-values on a coordinate system are the left-and-right values and
we can see that the parabola is expanding infinitely to the left and
to the right so the domain of the function is all real numbers.
Although there is an all real numbers option, you can also represent
this idea using a capital R like I have done below.