When using related list merge fields in a prompt template associated with the Account object inPrompt Builder, theActivities related listis not supported due to it being apolymorphic field. Polymorphic fields can reference multiple different types of objects, which makes them incompatible with some merge field operations in prompt generation.
* Option Bis incorrect because person accounts do not limit the availability of merge fields for the Account object.
* Option Cis irrelevant since even if no related lists are available at runtime, the prompt can still generate based on other available data fields.
For more information, refer toSalesforce documentationon supported fields and limitations inPrompt Builder.