What is required to configure an indirect lookup relationship in Salesforce between the Salesforce Opportunity records and those in a financial database?

Universal Containers wants to match Opportunity data from Salesforce to the records in a financial database.

What is required to configure an indirect lookup relationship in Salesforce between the Salesforce Opportunity records and those in a financial database?
A . Salesforce Record ID
B . TEXT(Id)
C . External ID
D . CASESAFE(Id)

Answer: C

Explanation:

An indirect lookup relationship is a custom relationship between two objects that are linked by an external ID field. The external ID field must be a custom field on the parent object that has the “External ID” attribute enabled. The child object must be a standard or custom object that is on the detail side of a master-detail or lookup relationship.