The correct answer is A because the code at line 03 creates an object by calling a constructor function with the new keyword. A constructor function must be declared with the same identifier used in the new Student( " David " , " Hamilton " ) expression. Inside the constructor, instance members should be assigned through this, because this refers to the newly created object. Option A correctly initializes this.firstname, this.lastname, and this.fullname, so beststudent.fullname is available at line 04 and evaluates to " David Hamilton " . Option B is invalid JavaScript syntax because var student(firstName, lastName) is not a valid function declaration. Option C is also invalid syntax for the same reason: var Student(firstname, lastname) cannot declare a constructor.
Option D uses a valid function declaration, but it assigns firstname, lastname, and fullname without this, which creates or references non-encapsulated variables rather than properties of the constructed object.
References/topics: JavaScript constructor functions, new keyword, object instance properties, this, custom classes.