A is correct: For a variable referenced in a custom prompt to be exposed as a mappable input in Power Automate, it must be explicitly defined as an input variable via the 'Add input' button or pane within the AI Hub prompt designer. Merely typing the placeholder in the prompt text is insufficient.
B is incorrect: Power Automate and AI Builder prompts support multiple input variables, allowing for complex prompts that consume various pieces of dynamic content.
C is incorrect: Custom prompts in AI Builder are generally flexible with data types that can be processed and often convert inputs to strings internally. This is not a primary reason for a field not appearing.
D is incorrect: While testing prompts is good practice, the lack of sample data for a specific input does not prevent it from appearing as a mappable field in Power Automate; it only affects the prompt's test execution.