Principles to apply: fairness and due process (no premature accusation), dignity and confidentiality, problem-prevention, and proportionality.
Assessing each action:
(3) States the problem without accusation and opens space for dialogue and fact-finding. This is the \emph{most appropriate} first step—respectful, non-threatening, and fair.
(5) A direct but calm inquiry can follow. Offering amnesty helps surface the truth and allows corrective action; while not ideal in all cases, it is more constructive than blaming or threatening. \emph{Second best}.
(4) Citing neighbours’ suspicions risks gossip and reputational harm; even with the caveat “I don’t think so,” it applies social pressure and weakens confidentiality. It may be used \emph{only after} private conversations fail. \emph{Third}.
(2) Accusatory and punitive upfront; violates due process and escalates conflict—\emph{inappropriate}.
(1) Ignoring the issue enables recurrence and fails the preventive goal—\emph{least appropriate}.
Thus the decreasing order of appropriateness among the given combinations is:
3, 5, 4, which matches option
D.
Final Answer:
\[
\boxed{\text{D. 3, 5, 4}}
\]