It would be
The second one down.
Looking at the first square in the example, let's call the top dot A and the lower one B.
Dot A travels down to the left and then up to the right. Think of it as an oscillating dot on that diagonal. Dot B appears to travel like a knight in chess, so in the second square, it is at top left, then bottom middle, then top right, and the only option from the right is middle left.