Graph Representation

Step 1: Understand the statements.
- **(A)** In a directed graph, the sum of lengths of all the adjacency lists is ** **. This is true because each edge is listed exactly once in the adjacency list of a directed graph.
- **(B)** In an undirected graph, the sum of the lengths of all the adjacency lists is ** **. Each edge is counted twice in an undirected graph, once for each endpoint.
- **(C)** For a dense graph, adjacency matrix representation is preferable. This is true because dense graphs have a large number of edges, making an adjacency matrix (which stores the presence of edges between every pair of vertices) more efficient than an adjacency list.
- **(D)** The memory requirement of the adjacency matrix of a graph is dependent on the number of edges. This is false; the memory requirement of an adjacency matrix depends on the number of vertices, not edges.

Step 2: Conclusion.
Thus, the correct answer is **(3) (A), (B), (C) and (D)**.