• CONCAT
• COLLECT
• SUM

The correct answer is: CONCAT

Explanation: Here is the official Amazon documentation reference for the CONCAT function in AMC SQL:

CONCAT
The CONCAT function concatenates multiple strings and returns the result as a single string. All inputs will be cast to string data type for joining.

CONCAT(expression [, expression]*)

You can find this directly in the Amazon Marketing Cloud SQL Functions documentation under String functions:

• AMC SQL Functions – CONCAT

This function can be used to append a string (such as a character or prefix) to an ID field (e.g., campaign_id), which helps prevent Excel from altering numeric IDs during import or formatting.