The incidence matrix of a graph is as given below

The incidence matrix of a graph is as given below—

The number of possible tree are—

Since the incidence matrix is complete because sum of every column is zero. Therefore, first make reduced incidence matrix, A_r

A_r =		–1	0	1	1	–1	0
A_r =		1	–1	0	0	0	–1

Number of tree = det
[A_r . A_r ^T]

= det		4		–1
		–1		3

= 12 – 1 = 11
Hence alternative (A) is the correct choice.