Unlike the range which describes all the y-values used by the graph, the domain describes the x-values used by the graph. The domain is basically describing how far left to right the graph goes.
What's tricky about this is to recognize that the graph goes on forever to both the left and the right. It never stops on either side.
In words, we'd say it goes from negative infinity to positive infinity.
As an inequality, we'd write this as Answer D.