5 is the correct scale factor
This is because there are 5 little segments that go from C to the larger point in the top right corner. We need 5 pieces of segment CB to get segment CB' where B' is that top right corner point.
Put another way: If the distance from C to B is 1 unit, then the distance from C to B' is 5 units. This shows the distance has multiplied by 5, and that's the scale factor.