How can a developer prevent IBM Bluemix from logging data in an API request to an IBM Watson service?

Setting a header parameter called X-Watson-Learning-Opt-Out with the value true in the HTTP request is the correct approach to prevent IBM Bluemix from logging data in an API request to an IBM Watson service. This specific header indicates the developer's intent to opt out of the data training process, meaning that data provided during the API call will not be used to improve the service's machine learning models.

Opting out can be crucial for developers who are handling sensitive or proprietary data and wish to maintain confidentiality or comply with specific data governance regulations. By including this header in the request, the developer ensures that any information sent will be excluded from the datasets used for training purposes.

The other options do not effectively stop data from being logged in the same way. Modifying the IBM Bluemix dashboard does not provide an immediate effect for individual API requests, and passing parameters within the request body may not influence the logging behavior. Meanwhile, using headers that are not explicitly designed for this purpose might not yield the desired outcome regarding data privacy in training contexts.