B is correct.

Roll up only available with master-detail relationship.

they said that "serviceJob-o, with an optional Lookup field to Account called partner-service-Provider_c." which means lookup relation (I guess) and roll-up summary fields will not be available for lookup relation so the other available options are either flow or trigger among which flow is easy to implement and most appropriate flow is record trigger flow.

Correct answer D.
While a record-triggered flow could be used to update the TotalJobs__c field whenever a ServiceJob__c record is created or updated, it would require additional setup and maintenance. This method is more complex compared to using a roll-up summary field, which provides automatic updates without additional configuration.

An Apex trigger would also update the TotalJobs__c field whenever related ServiceJob__c records are changed. However, using Apex code for this purpose is more complex and less efficient compared to a roll-up summary field. It involves writing and maintaining code, and handling bulk operations and governor limits.

Therefore, the most efficient and straightforward approach is:
D. Change TotalJobs__c to a roll-up summary field.