For displaying personalized content in a newsletter, which option should be used if subscriber data isn't in the send definition?

AMPscript is the correct choice for displaying personalized content in a newsletter when subscriber data isn't predetermined in the send definition. This scripting language is specifically designed for Marketing Cloud and allows for dynamic personalization and content personalization based on subscriber attributes.

When using AMPscript, you can pull data directly from the subscriber's profile or other data extensions. This flexibility enables you to create tailored experiences that can vary for each recipient, enhancing engagement and relevance. Essentially, AMPscript facilitates the retrieval and display of individual subscriber data dynamically, which is critical for personalization efforts within newsletters.

Personalization strings also offer a way to insert subscriber-specific information, but they rely on the assumption that this information is already available within the context of the send definition. If subscriber data is not included in the send definition, then personalization strings would not function effectively in pulling in the needed information.

The Guide Template Language and SQL Query are important tools within Marketing Cloud, but they do not directly serve the same purpose for dynamic content in newsletters as AMPscript does. Therefore, AMPscript stands out as the most effective option for conditional logic and personalized content display when subscriber data is not present in the initial setup.