E is the answer to this question. There are some people who would need to know how to make this work because this is a new field type. This is normally used in order to link the external object to a custom object. This is the same with how a regular lookup relationship can be added to different custom objects.
This is normally used when the external data does not have the right Salesforce record IDs. It is important that you will understand the different ways on how to do this because this is not too complicated.