Note: To manage resources and ensure disposal. Analysis: The using statement is used to ensure that IDisposable objects are disposed of properly, automatically releasing resources when they are no longer needed.

Note: Polymorphism in C# can be achieved through method overloading (compile-time polymorphism) and method overriding (runtime polymorphism), both of which can involve inheritance and abstract classes.

Note: It allows a method to be overridden in a derived class. Analysis: The virtual keyword is used in a method declaration to indicate that the method can be overridden in any derived class, enabling polymorphism.

Note: It makes a field immutable after initialization. Analysis: The readonly keyword allows a field to be assigned only during declaration or in the constructor of the class, making it immutable thereafter, but unlike const, it can be set at runtime.

Note: int[] arr = new int[5]; Analysis: This is the correct syntax for declaring an array of integers in C#. The other options either use incorrect syntax or are invalid in C#.

Note: The x++ operator is a post-increment operator, which means it returns the current value of x (10) and then increments it after the statement is executed.

Note: It prevents a class from being inherited. Analysis: The sealed keyword is used to prevent further inheritance of a class, meaning no other class can derive from it. It can also be used to prevent overriding methods in a derived class.

Note: A constructor is a special method that is called when an instance of a class is created. It is used to initialize the object's data members.