When creating a custom async validator for a form control, which type should the validator return to indicate a validation error?

When creating a custom async validator for a form control in frameworks like Angular, the validator should return an Observable to indicate a validation error. This allows you to handle asynchronous validation, such as making HTTP requests to validate the input. Observables provide a convenient way to handle async operations and emit error values when validation fails. Using other types like numbers, strings, or booleans would not be suitable for asynchronous validation scenarios.