Find the missing number?

Answer: Option A

Starting with the diagram in the top left, and moving to the right, ending up with the bottom diagram, average the values in the top two boxes and put the result in the lower box, adding 1 for the first diagram, 2 for the next, then 3, then 4. So,

X = {(8 +6)/2} + 4 = 7 + 4 = 11.