Final answer:
Category II codes in the CPT coding system follow a format of a single letter followed by four digits, used for tracking performance measures.
Step-by-step explanation:
In the Current Procedural Terminology (CPT) coding system, Category II codes are used to track performance measures and consist of a structure that involves a single letter followed by four digits. Thus, the correct structure for Category II codes in CPT is A single letter followed by four digits. These codes provide an optional but standardized coding system for reporting performance measurement data in addition to regular Category I codes.