What is the correct syntax for a Logger component to output a message with the...

Option 3 is the only corect answer as it concatenates payload with String.

Below option wont work.

#["The payload is " ++ payload]

Concatenation function expects both arguments to be string. As the question says payload is json object , this will throw error while running it. You can try this in Anypoint Studio and you will get the same result which I mentioned.

hence correct answer is

The payload is: #[payload]