Fertilization lies somewhere in the B to C range. Once it happens, the
normal course of development is to produce an adult human, whereas gametes
alone just die. Thus, it seems like the best biological cutoff to me. I
don't know at what point the soul would first be present (if it's a point
at all and not more gradual), but it seems possible that it's there by C.
Both normal reproduction and cloning would yield a new individual with a
soul starting from existing folks with souls, so I don't see possible
twinning or fusion as a problem.