To create an HTTP GET request that connects to an external REST service in a Business Central online environment, the solution to use is the HttpClient data type variable (E). The HttpClient data type in AL language is designed for sending HTTP requests and receiving HTTP responses from a resource identified by a URI. This makes it the ideal choice for interfacing with external REST services, as it provides the necessary methods and properties to configure and execute HTTP GET requests, handle the responses, and process the data returned by the REST service. This approach is more direct and flexible compared to using specific codeunits like "Web Request Helper" (B) or "Http Web Request Mgt" (D), which might not provide the same level of control or specificity needed for RESTful interactions.