Final answer:
The tier required for continued secret clearance eligibility is Tier 3, which pertains to the periodic reinvestigations needed to maintain access to classified information.
Step-by-step explanation:
The tier required for continued secret clearance eligibility is Tier 3. This is part of the background investigation process which is essential for determining eligibility for access to classified information. The investigation tiers have replaced the previous system of Single Scope Background Investigation (SSBI) and other types of investigations, streamlining the process into a tiered structure. Tier 3 investigations are often referred to as "reinvestigations" which are required periodically to maintain the clearance. It’s important for those holding or seeking a secret clearance to be aware of the requirements and periodicity of such investigations to ensure they don't encounter any lapse in their clearance status.