Which of the following ratings does NOT require a security clearance?

The rating of Machinery Technician (MM) does not require a security clearance because the tasks and responsibilities associated with this rate typically involve maintenance and operation of machinery and mechanical systems onboard naval vessels, which do not usually require access to classified information.

In contrast, other ratings like Operations Specialist (OS) and Information Systems Technician (IT) require security clearances due to their involvement with classified information and data systems. Operations Specialists work with sensitive information regarding naval operations and tactics, while Information Systems Technicians handle secure networks and communications. The Yeoman (YN) rating also requires a clearance since they manage personnel records, administrative processes, and documents that may contain sensitive information.

Therefore, the designation of Machinery Technician stands out as it does not align with roles that generally engage with classified data or environments, making it the correct answer in this context.