Solution

The correct answer is 2l

Explanation:

In an undirected graph G(V, E) , the degree of a vertex is defined as the number of edges incident to that vertex. 

The Handshaking Lemma:
According to the Handshaking Lemma in graph theory, the sum of the degrees of all vertices in an undirected graph is equal to twice the number of edges. This is because each edge contributes to the degree of two vertices.

• Mathematically, this can be expressed as: \(\text{Sum of degrees of all vertices} = 2 \times \text{number of edges} = 2l\)
• where l is the number of edges in the graph.

Conclusion:
Therefore, the correct answer is: 1) 2l