The answer isn't correct because "it depends"!

I answered it correct but only because of the standard settings for the session parameter [CONCAT_NULL_YIELDS_NULL].

SET CONCAT_NULL_YIELDS_NULL OFF;

GO

SELECT session_id, concat_null_yields_null FROM sys.dm_exec_sessions WHERE session_id = @@spid;

-- now it is the default behaviour and the expected answer of QotD

select concat(null,'testString') as a

, null+'testString' as b

SET CONCAT_NULL_YIELDS_NULL ON;

GO

SELECT session_id, concat_null_yields_null FROM sys.dm_exec_sessions WHERE session_id = @@spid;

-- now both concatenations will return the same result!

select concat(null,'testString') as a

, null+'testString' as b

As others said - good question because of the amount of failed answers. But this - little - issue is worth to be mentioned 🙂