In Prim's Algorithm, first a min heap is created (Build min heap) with the given nodes, such that the source node (chosen randomly) gets a key 0, and all other nodes get keys infinity.
At each interation, the node with the least key is deleted (Delete-min operation), and the keys of all the nodes adjacent to it are suitably reduced (Decrease Key).
By going with the above method, in the 4th sequence, 'd' is chosen and assigned key 0. Adjacent to 'd' are 'c', 'e', 'f'. Keys of 'c', 'e', 'f' are reduced to 2,3 and 1 respetively. In the next iteration, 'f' is chosen (f is minimum). Adjacent to it are 'c', 'e', 'b'. Keys of 'c', 'e', 'b' are made as 2,3 and 2 respectively. Now, 'c' and 'b' have the least keys.
So, the sequence should have been (d,f),(d,c)...... or (d,f),(f,b)........ Hence sequence 4 is wrong.
(Here I have followed the algorithm of Cormen)
