Given graph will be having 6 or 7 as the weight of its MST.
We will get weight 6 when there will be no cycle formation during picking of edges weight 1,2,3 and thus 1+2+3 = 6.
But in the worst case, we will be having cycle formation when we pick edge having weight 3 and thus, in that case, we'll have to leave weight 3 and pick edge with weight 4 (always).So this will give weight 1+2+4 = 7.
Since we are having two variants of MST having weight 6 and 7, we have to choose maximum among them.
So the answer is 7.