The configuration that most likely caused the email send to fail in this scenario is:

B. Incorrect Sending Relationship

The sending relationship defines how the system should send emails to the recipients. If the sending relationship is set up incorrectly, it can prevent emails from being sent to the intended recipients. It's crucial to ensure that the sending relationship is properly configured to send emails to the specified group of customers. The other options, such as having two fields with 'EmailAddress' data type or failure to choose the send classification, might also cause issues but are less likely to be the primary cause of the send failure in this context.

Incorrect Sending Relationship

C is true