Final answer:
You define a validation for the combination of a general ledger account and cost center at the Document Item level to ensure each posted item adheres to predefined rules.
Step-by-step explanation:
When defining a validation for the combination of a general ledger account and cost center, you typically consider the context in which the validation rules need to be applied. In most enterprise resource planning (ERP) systems, validations can be set up at different levels of the document process. For general ledger account and cost center combinations, the validation is often needed at the line item level to ensure that each item posted is valid according to the predefined rules and combinations. Therefore, you would define this validation at the B) Document Item level. This allows for the application of validation rules at the most granular level where individual account postings are made against cost centers.