Final answer:
The document used to assign direct labor to each job in job order costing systems is known as a (Option a) job sheet or time ticket, which helps in the precise assignment of labor costs to specific jobs for proper job costing.
Step-by-step explanation:
The source document used to assign direct labor to each particular job is known as a time ticket or a job time ticket. While the options provided, such as job sheet, labor ledger, work order, and payroll register, are all related to tracking labor and job costs, the specific document that is normally used to record the amount of time an employee spends on a particular job and then assign those labor costs directly to jobs is a time ticket. This document is crucial in job order costing systems where individual jobs or batches require specific amounts of labor. It provides accuracy in assigning labor costs to the correct job, thereby allowing for precise job costing and project management.