answer correct for a brainliest and also a ! don't answer if you don't know it .

The domain is all the numbers of which the inputs can be put. Here, we have a graph, which simplifies things. We just need to look at the x-values of the graph, or the horizontal line. The graph extends both negatively and positively. Thus since the graph extends both ways (right and left) forever, the answer would be D, or all real numbers.