This graph is not a tree because it is not connected.
Work Step by Step
A tree is a graph that is connected and has no circuits. A graph is connected when there is a path between every pair of vertices. Clearly the graph in this question is not connected. For example, let's consider vertex A and vertex D. There is no path in the graph which connects vertex A and vertex D. Therefore, this graph is not connected. Since this graph is not connected, this graph is not a tree.