Which of these lists are arithmetic progressions?

Step 1. (a) The differences are $6, 6, 6$. An AP with $d = 6$.

Step 2. (b) The differences are $3, 5, 7$. They are not equal, so it is not an AP.

Step 3. (c) The differences are $-4, -4, -4$. An AP with $d = -4$.

Step 4. (d) The differences are $0, 0, 0$. An AP with $d = 0$.