Acting diligently to ensure quick service is the definition of:

The term that best defines acting diligently to ensure quick service is "due diligence." This concept refers to the level of care and effort that process servers must apply to fulfill their responsibilities in a timely manner. Due diligence implies that a process server is actively taking the necessary steps to locate and serve legal documents, rather than simply going through the motions. It emphasizes the importance of being proactive and thorough, ensuring that the service of process is handled efficiently and effectively. This is crucial in maintaining the integrity of the judicial process and upholding the rights of all parties involved.

In contrast, the other options either do not specifically address the diligence aspect or focus on different aspects of the service process. "Effective service" speaks more to the outcome of the service rather than the diligence required in the process. "Expedited service" suggests speeding up the process but does not inherently emphasize the diligence involved in preparing for such service. "Proper protocol" refers to the adherence to established procedures, which is important but does not capture the proactive nature of diligence needed to ensure timely and effective service.