Explanation
To import data into a non-sendable data extension without a primary key, the admin could use the Overwrite import type3. This import type will delete all existing records in the data extension and replace them with the new records from the import file. This import type does not require a primary key because it does not need to match or update any existing records. The other options are incorrect because:
Add Only. This import type will add new records to the data extension without affecting existing records. However, this import type requires a primary key to prevent duplicate records from being added3.
Add and Update. This import type will add new records and update existing records in the data extension based on the primary key. However, this import type requires a primary key to match and update the existing records3.
Update Only. This import type will update existing records in the data extension based on the primary key. However, this import type requires a primary key to match and update the existing records3.