Which MCO governs HRST operations?

The choice identifying MCO 3500.42C is correct because it represents the most current directive governing HRST (Helicopter Rope Suspension Techniques) operations as of the last training date. Military and operational directives are often updated to reflect new standards, practices, and safety protocols. Each version of an MCO typically builds upon the previous one, correcting issues or incorporating new policy changes that enhance operational effectiveness and safety.

MCO 3500.42C would contain the relevant guidance, procedures, and standards that ensure personnel are trained and equipped to carry out HRST operations safely and effectively. Other versions, such as A, B, and D, would either be outdated or discuss topics not specific to the current regulation of HRST operations, making option C the definitive choice for this context.