The correct answer is D. You must not use the machine after 5.30 p.m.
A. You can’t use the machine after 5.30 p.m. is a negative statement, but it does not express a strong prohibition. It could be interpreted as a suggestion or a recommendation, rather than a rule.
B. You mayn’t use the machine after 5.30 p.m. is also a negative statement, but it is even weaker than A. It could be interpreted as a polite request, rather than a rule.
C. You need not use the machine after 5.30 p.m. is a positive statement, but it does not express a strong obligation. It could be interpreted as a suggestion or a recommendation, rather than a rule.
D. You must not use the machine after 5.30 p.m. is a negative statement that expresses a strong prohibition. It is the only option that clearly states that using the machine after 5.30 p.m. is not allowed.