b - is correct because maceo lenses designed for bellows use aren't set back like this, but for focussing 1:1 etc you need a long helical.
and . . . .
a - is correct as well look how far set back an f2.8 50mm Tessar is, smaller diameter but also far smaller overall physical length of the optical cells.
Also some macro lenses have built in correction to maintain a constant aperture as the focus closer.