You know what, this isn't a helpful response.
The OP is doing exactly what the ethical hunter should. Evaluating all his options to make the cleanest most ethical kill and trying to improve his skills.
Moreover, he is planning for the real world where things go bad sometimes. He wants the option with the most room for the unexpected (I won't say "error" because you'll just spout off about "go to the range more"). Show me someone who claims to make a perfect shot every time he pulls the trigger and I'll show you a damn liar.
Insisting that "He ought to already know" is both narrow minded and self-congratulatory. Perhaps the only "acceptable" way to learn to hunt is from one's "Pappy and Gran-Pappy on the farm"??? If a guy didn't grow up in a hunting household, well I guess he's just SOL. Goodness knows it is a crime against nature for someone to try to learn from the internet.
And in case you didn't notice - the OP already knows damn well where the vital organs lie - look at the picture he. Blue ovoid for lungs, red ovoid for heart, gray ovoid for liver.
Of the two, I like A better, not because the animal will be 'more dead' but because a miss 'low" at B is likely to break a leg - slow and painful demise. A low miss at A may still be painful, but less likely to be crippling.
my 2 cents
Fat