Answer: 2
Usually a rotation doesn't happen with dilation problems, but it appears that has been done here. Anyway, note the short leg of the left triangle is 1 unit. Compare this to the short leg of the triangle on the right which is 2 units. The jump from 1 to 2 means we multiplied by 2. The scale factor must be 2.
If you focus on the long leg of the left triangle, it is 2 units while the long leg of the triangle on the right is 4 units. This also represents a scale factor of 2.