Count the dots on each half of the dominoes. Which domino should come next?

The left half gains one dot each time (1, 2, 3, 4) and the right half loses one (5, 4, 3, 2), so the total stays 6. Next: 5 on the left and 1 on the right: (b). (a) forgets to add to the left half, (c) forgets to take from the right half and (d) is the first domino again.