Put the values 3,4,5,6...to check using which value the options are giving different anwers, then using the least such value, draw the required graph. In this case n=5 will suffice. You will see that the cost of MST is 21 which is satisfied only by option B. Having found the method you will know where to attach the new node, V6. Then from the new MST find the value of v5 to v6 which will be nothing but 21 + the minimum value to reach to v6 which will be (v4 to v6)=10, thus the answer will be 21+10=31.