Final answer:
The factor that determines the length of A'B' relative to AB in the dilation is the scale factor of dilation.
Step-by-step explanation:
The factor that determines the length of A'B' relative to AB in the dilation is the scale factor of dilation (B). The scale factor is a ratio that compares the lengths of the corresponding sides of the dilated image to the original figure. If the scale factor is greater than 1, the dilated image will be larger than the original figure. If the scale factor is between 0 and 1, the dilated image will be smaller than the original figure.