In which one of the above figures is ab=ac

If it was just for random figure, then compare the original figure with the other figures. Take the other figure and put them in the same direction as the orignal one. For example if there was a large square, and then a small square put in the position as a diamond and a rombus. Flip the small diamond to look like the square. Flip the rombus and diamond until they look like they have similar sides as the orginal large square. You will find the answer is the small square. Hope this helped. If not, tell me and see if I can think of a better easier example. :)