Final answer:
The key performance measure relating to access of care in ambulatory care improvement assessments is the average telephone wait time.
Step-by-step explanation:
When assessing key performance measures in the ambulatory care process improvement assessment, average telephone wait time relates to access of care. This metric is a direct measure of how quickly patients can reach healthcare providers or services by phone, which is an important aspect of healthcare accessibility. In contrast, FTE support staff per MD relates to resource allocation, rejection/denial rate of insurance pertains to billing and coverage issues, and immunization rates are indicative of preventative health measures.