To ensure that server tasks run sequentially, or "one-at-a-time," the execution mode must be set to serial. Serial execution processes tasks one after another, where a subsequent task only begins after the preceding one has completed. This is in contrast to parallel execution, where multiple tasks run concurrently. In CRM Analytics, this can be achieved by scheduling jobs to run based on an event, such as the completion of a previous data sync or recipe, effectively creating a serial chain of execution.

B. Set Default priority to 0: Priority influences which job runs first when multiple jobs are queued, but it does not prevent them from running concurrently if resources are available.

C. Set Frequency to Hourly: Frequency determines how often a schedule is initiated (e.g., hourly, daily). It does not control how the tasks within that schedule are executed (serially or in parallel).

D. Set Execution to Parallel: This is the opposite of the requirement. Parallel execution runs multiple tasks simultaneously, not one-at-a-time.

1. Salesforce Help, CRM Analytics Documentation, "Schedule Data Sync and Recipes to Run After Each Other": This document explains the concept of event-based scheduling to create dependencies. It states, "To run a recipe or dataflow after the sync for a specific connection completes, schedule them to run after the connection syncs. You can also chain jobs to run after a recipe or dataflow finishes." This chaining is the mechanism for enforcing serial execution.

2. Salesforce Help, CRM Analytics Documentation, "Schedule, Run, and Monitor Analytics Jobs": This guide details scheduling options. In the context of data sync, it mentions, "To run a connection’s sync jobs one after another, you can schedule them in serial." This directly confirms that "serial" is the correct term for "one-at-a-time" execution.

3. Salesforce Help, CRM Analytics Documentation, "Data Integration Guide", Chapter: Run Data Integration Jobs: This guide discusses job scheduling and dependencies. It notes, "To ensure that a dataflow has the latest data from a particular Salesforce object, run the dataflow after the data sync for that object is complete. You can schedule a dataflow to run after data sync." This describes a practical application of serial execution.