What are the different types of relationships for external objects? Choose three answers.

Answer options:

A.Lookup relationship
B.External lookup relationship
C.Indirect lookup relationship
D.Internal lookup relationship

Correct Answer: A, B & C 
The three types of relationships for external objects are lookup, external and indirect lookup relationship. 
Option D is incorrect because internal lookup relationship doesn’t exist in salesforce. 

https://help.salesforce.com/articleView?id=external_object_relationships.htm&type=5