Answer
AD, DE, and DF are bridges.
Work Step by Step
For any two vertices in a graph, if there is at least one path which has these two vertices as endpoints, then the graph is connected. If removing an edge makes the graph disconnected, then that edge is called a bridge.
The graph in this question is connected.
If we remove edge AB, edge BC, or edge AC, the graph is still connected. Therefore, these three edges are not bridges.
However, if we remove edge AD, edge DE, or edge DF, the graph will not be connected. Therefore, these three edges are bridges.
AD, DE, and DF are bridges.
