First we draw a curve around the tree starting at the root, moving along the edges and terminate at the root.
Now for the post order traversing, we list the vertices last time the curve passes.
Here those vertices are in order 4>>5>>2>>6>>7>>3>>1...
Since 4=a, 5=b, 2=-, 6=c, 7=d, 3=*, 1=+
This gives A as the answer...