"A" is the outside diameter of the surround - to the very edge of the speaker gasket.  If the "A" dimension on our replacement surround is a little too large, you can trim off a little from the new surround with a scissors (perhaps 1/8").

"B" is the inside diameter of the surround - measured right across the center of the speaker.   This is a very important dimension, so measure carefully. 

"C" is the diameter of the dust cap.   You have some choice on the type and size of dust cap you choose.   Sometimes you may want to choose a dust cap that is slightly larger than your old one.   That's perfectly OK!   Never choose a dust cap that is smaller than the old one, however.

  

Look at how the old foam surround attached to the cone.   Some surrounds are glued to the cone on the angled edge, as shown to the left.   This is called an angle surround.  
Other surrounds attach to the cone along a thin flat edge on the outside of the cone, as shown to the left.   The type of surround that attaches in this way is called a flat surround.