Which option is not a valid step to fetch cities for a selected country in a dropdown?

The step that is not valid for fetching cities based on a selected country in a dropdown is the one that suggests calling the GetCities Aggregate in the GetCountries' On After Fetch Event handler. In a typical reactive application, the process of fetching related data, such as cities based on a country's selection, is usually executed in response to user interactions, specifically when the user selects a country from the dropdown.

The On After Fetch Event handler is intended for actions that should take place after the data for countries has been successfully fetched, not for triggering other data fetch operations like obtaining cities. To effectively get cities associated with the selected country, actions should be aligned to the user’s selection and interact with the relevant aggregate directly upon that interaction, such as when the country dropdown changes.

The other options represent valid steps in the process. Setting the Fetch property to "Only On Demand" allows the GetCities aggregate to only fetch data when explicitly commanded, which conserves resources. Refreshing the GetCities aggregate in the OnChange Action of the country dropdown ensures that the cities are updated whenever a user selects a different country. Filtering the GetCities aggregate based on the selected country establishes the necessary connection between the two data sets, allowing only relevant cities to be fetched for the currently