According to the OmniStudio DataRaptor Extract documentation, if a developer wants to limit the number of records returned, the best way is to set the "Limit" property on the element or action that is calling the DataRaptor, such as in an Integration Procedure or OmniScript Action.
* A. Set the Limit property on the action that calls the Data Mapper Extract - This is the correct method. The calling context (e.g., OmniScript Action or Integration Procedure) includes a property for setting the limit on query results, which is a recommended and clean method to ensure only one record is retrieved.
Incorrect Options:
* B. There is no "LIMIT" clause within the DataRaptor Extract definition UI or JSON definition itself.
* C. Custom Output Type controls the structure, not the quantity of output.
* D. The Filter function applies conditional logic, not record count.
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Salesforce OmniStudio Developer Guide # DataRaptor Extract Configuration