Based on the code provided, here are the necessary changes:
Change B is required because the argument in the Apex controller's serverEcho method must match the data type expected by the JavaScript calling the method. The Apex method expects an Object named firstName, which is then cast to a String inside the method. The JavaScript is passing an object with a property firstName, so no change is needed in the JavaScript.
Change D is correct because line 06 is redundant and unnecessary. We can directly use firstName in the return statement as it is already a String type. The get() method is typically used when you have a Map and you need to retrieve a value by its key. Since firstName is already being passed as a String, there's no need to cast it or retrieve it using get().
Apex Developer Guide