This is a classic case of trigger recursion caused by the Salesforce Order of Execution. When an Account is updated to "Customer," the following occurs:

The original "Before" and "After" triggers run. The After trigger creates the first Account Plan.

The Record-Triggered Flow executes and updates the "Customer Since" date field.

Because a Flow (or Workflow) performed a field update, Salesforce re-triggers the Apex triggers on the Account object to ensure any logic dependent on that new date field is executed.

During this second pass, the After trigger runs again and, seeing the status is still "Customer," creates a second Account Plan.

To prevent this, developers use a static variable (Option D) in a handler class to act as a "recursion guard." Static variables persist for the duration of the entire transaction. By checking this variable at the start of the trigger, the code can detect that it has already run once and skip the record creation on the second pass. Option B and C describe standard Flow behavior but are not the "fix" for the trigger logic. Option A would cause issues with limits but wouldn't necessarily cause a double-fire unless recursion was involved.