I like to think of the domain and range as coordinates, so think of the domain as the x value, or the input, and the range as the y value, or the output.
So, b wouldn't work because of what I just explained.
C could be defined as slope.
And d won't work because this would be defined as a function.
So the only answer left would be A. The inputs of a relation (but just know that domain is also the input of a function)
I hope I helped!
