To answer this question, we need to understand the concept of null reference types in programming.
In programming languages that support null reference types (such as C#), a null reference represents the absence of an object. When unboxing is performed on a null reference type, an exception is thrown.
Option A) true - This option is incorrect because unboxing a null reference type does not return a null. Instead, it throws an exception.
Option B) false - This option is incorrect because unboxing a null reference type does not return false. Instead, it throws an exception.
Option C) an exception is thrown - This option is correct. Unboxing a null reference type will result in an exception being thrown.
Option D) None of these - This option is incorrect because option C is the correct answer.
Therefore, the correct answer is C) an exception is thrown.