The given sequence is: B @ C 7 N R % 5 $ G 6 K M & 4 S # P U 5. Applying the rule that each symbol swaps places with the consonant right after it, @ (before C), $ (before G), and # (before P) each swap with the consonant that follows them.
After making these swaps, the sequence becomes: B C @ 7 N R % 5 G $ 6 K M & 4 S P # U 5. Counting from the right end: 1st is 5, 2nd is U, 3rd is #.
So the element third from the right is #, matching option 2.