Spoilers for all paths up ahead:
I figured out what they mean with Path C. It means there are 3 possible paths in Shoichi's route. Path C is the bad route where you fail to convince Sho to visit his mom, thus ending up quarreling with him. Path A is the good route where you convince Sho to visit his mom. Path B is where you dont push him to visit his mom, thus never going to talk to her.
You get Path B by just choosing to stop pushing him. You can get Paths A and C by choosing to keep pushing him, which will then depend on your last 2 choices. (Hint: go big or go home.)