hey everyone follow this simple approach,think like this
for every value of n we can have number of graphs,ok different-different kind we say, for ex n=3 only one graph K3 we will have,for n=4 ,2 diff kind of graph we have i.e. cycle of 4 and a K3 with one edge connected via bridge to one vertex.
like that we can draw a no of diff-diff kind of graphs,but notice one thing which is imp here
in worst case,we will always have kind of graph with k3,along with single single edges -edges as bridge, and that k3 will will always result in atleast 3 diff spanning trees,
conclusion- no matter what kind of graph you have drawn,that k3 will always be there in one graph and will give atleast 3 diff spanning trees.therefor answer is 3.
any corrections,suggestions are welcomed.