Final answer:
The correct statement to store a null value in a variable with nullable reference types enabled is 'b. string? description = null;'. The '?' indicates that the variable is a nullable reference type, allowing null assignment.
Step-by-step explanation:
If nullable reference types are enabled in a programming environment that supports them (such as C# starting with version 8.0), the correct way to store a null value in a variable specifically designed to hold nullable references is by declaring the variable with a question mark after the type. The correct statement from the provided options is:
- b. string? description = null;
The question mark (?) after the type string indicates that the variable description is a nullable reference type. It allows for null to be explicitly assigned to it without generating a compiler warning or error, which would otherwise happen if nullable reference types were enabled and you tried to assign null to a non-nullable reference type.
Option a would normally be acceptable in code where nullable reference types aren't enabled or being enforced. Option c is not valid syntax in C# for nullable reference types, as the exclamation mark (!) is used for null-forgiving operator, not for declaring a variable. Option d is also incorrect; it appears to be a mix-up and does not represent valid C# syntax for assigning a null value to a variable.