I understand why A is right but I would 100% get this wrong without being walked through it

@TheBigFatPanda JY was right: the correct direction is Contributes to the public good → Morally right.

The statement says that what makes actions morally right is their contribution to the public good. In other words, contribution to the public good is sufficient for, or guarantees, moral rightness.

The other direction, Morally right → Contributes to the public good, would mean every morally right action guarantees a benefit to the public good.

But the original claim doesn’t say that. There may be cases of actions being morally correct for reasons unrelated to the public good. All the original statement guarantees is that if you contribute to the public good, that action is morally right.