The principle says a court's decision is null and void if that court did not have jurisdiction over the subject matter of the dispute. A obtained a decision from a court that lacked jurisdiction to deal with the subject matter. The question is whether that decision can still be enforced.
Since the court that decided the matter had no jurisdiction over its subject matter, the principle marks the decision as null and void, which means it cannot be given any enforceable effect.
Therefore, the correct answer is Decision cannot be enforced because decision is null and void.
The single fact that controls this question is that the court which decided the matter did not have jurisdiction over the subject matter at all. Checking each option against that one fact, rather than against anything else in the scenario, resolves the question quickly.
Since none of the other three options actually respond to the missing subject matter jurisdiction, only the option built around that exact defect survives.
Therefore, the correct answer is Decision cannot be enforced because decision is null and void.