Hey there!
In order to see if a line is a function or not, we can use the straight line test. That means we can throw a straight line right through the middle and it would only cross one point. If it crosses more, it's not a function.
Based on this rule, it's reasonable to say that D is the answer. You could throw a line right in the middle, and it would go through those two circles. Also, it's not practical to graph that anyway, it's not even connected.
Hope this helps!