Edge DF is a bridge because if we remove this edge from the graph, then the graph will be disconnected.
Work Step by Step
If a graph has two vertices such that there is no path with these two vertices as endpoints, then the graph is called a disconnected graph. A bridge is an edge which causes a graph to become disconnected if the edge is removed. Initially, the graph is connected because for any two vertices, we can find a path which has these two vertices as endpoints. Let's suppose that edge DF is removed from the graph. Let's consider the vertices E and F. Without edge DF, there is no path which has vertices E and F as endpoints. Therefore, the graph is disconnected. Edge DF is a bridge because if we remove this edge from the graph, then the graph will be disconnected.