Which options can be considered for integrating customer data into an external marketing system?

Using a custom Apex web service to retrieve customer data is a viable option for integrating data into an external marketing system. This method allows for real-time or scheduled access to Salesforce data, providing flexibility in how data can be accessed and shared. By creating a web service in Apex, you can define specific endpoints to expose the customer data that is needed by the marketing system, ensuring that only the required information is sent, which can help maintain data security and integrity.

Additionally, a custom web service can handle complex business logic, perform data validation, and provide a tailored response based on the requirements of the external system. By utilizing the Salesforce platform's capabilities, this option can effectively meet the integration requirements, especially when a more controlled or customized approach is necessary for data sharing.

While the other options also provide methods for integration, they may not be as customized or direct as using a web service, which can limit flexibility in certain scenarios.