$\underline{\mathbf{Answer:B}}\Rightarrow$
$\underline{\mathbf{Explanation:}}\Rightarrow$
Make a square of $4$ vertices and make it as a complete graph, that is, having $6$ edges.
Now, make the spanning tree. It will be the $3$ adjacent edges and the weight will be $2+2+2=6$.
Now, check the options and substitute $\mathbf {n = 4}$, then only option $\mathbf B$ is satisfied.
$\therefore \mathbf B$ will be the answer.