You need to create an alternate key for the Dataverse table. 
Please select all correct statements about what you can do with the alternate keys.

Answer options:

A.You can create an alternate key on the column with duplicated data in multiple records
B.You can define an alternate key as a unique combination of the multiple columns
C.You can delete the primary key after you create an alternate key that becomes a primary key
D.You can create not more than ten alternate keys per table
E.You can use alternate keys for integration with the external databases

Correct Answers: B and E 
All Dataverse tables have a primary key. A primary key is a GUID. When you read or write records, it is not efficient to search for GUID first and then access the record. Therefore, it is more convenient and productive to have another unique key for the record — an alternate key. You can use any column or combination of columns that uniquely identify the records as an alternate key. An alternate key can be used as a substitute for a primary key. The alternate keys are important for integration between Dataverse records and external sources. External databases can use an alternate key for the records access. Option B is correct because an alternate key can be a column or combination of columns that uniquely identifies the Dataverse record.
Option E is correct because external databases can use an alternate key for the records access.
Option A is incorrect because the Dataverse will not allow you to create an alternate key on the column that cannot uniquely identify the record.
Option C is incorrect because you cannot delete a Primary key that the Dataverse generates automatically. The primary key is a GUID and uniquely identifies each record. The alternate key is an alternative to the GUID of the primary key.
Option D is incorrect because users can create not more than five alternate keys per table.
For more information about Dataverse alternate keys, please visit the below URLs: 

https://docs.microsoft.com/en-us/learn/modules/create-manage-fields-within-entity/7-alternate-key
https://docs.microsoft.com/en-us/powerapps/developer/data-platform/define-alternate-keys-entity
https://docs.microsoft.com/en-us/powerapps/maker/data-platform/define-alternate-keys-portal