The rule here has a single trigger: subject matter jurisdiction. If the court deciding a dispute did not have the power to hear that type of dispute in the first place, the principle labels whatever it decided as null and void, with no exceptions carved out for the presence of parties or the formal status of the body as a court.
On these facts, A got a decision from a court that admittedly lacked jurisdiction over the subject matter. That single fact is enough to trigger the principle, because the rule does not ask whether the parties attended, or whether the body was recognisably a court, it asks only whether subject matter jurisdiction existed.
A void decision carries no legal weight, so there is nothing left to enforce. Every option that tries to enforce the decision on some other basis, such as attendance or formal courthood, is answering a question the principle never asks.
So the correct answer is Decision cannot be enforced because decision is null and void.
Testing each option here by asking what would have had to be different for it to be true makes the answer clear.
Only the option that requires no change to the facts, the one built on the decision being void, actually fits what is described.
Therefore, the correct answer is Decision cannot be enforced because decision is null and void.