Effective discipline combines respect for students with clear and consistent boundaries.
Why D fits: A polite but firm response communicates expectations without humiliating students.
Why the others are weaker: Allowing every behaviour can undermine learning; excessive strictness may create fear. Occasional problem-solving activities are useful but do not by themselves establish classroom discipline.
In practice: Explain rules, apply consequences fairly, listen to students, and teach the behaviour expected. Discipline should support a safe environment for learning.