Which pair of rectangles is similar? a b c d

B) the one that has the sides 4 & 8 and 3 & 6.  

The length on both of these rectangles are twice as long as the width

one rectangle goes from a width of 4 and its length is doubled to a length of 8
it is similar to the 3 & 6 because the width of 3 is doubled to get the length of 6

hope this helps