D. Exception classes must end with the word exception.

Creating custom exception classes in Apex requires extending the built-in Exception class and naming the class with the suffix “Exception.”

B. The default modifier for a class is private.

A top-level class in Apex cannot be private, but for inner classes, if no access modifier is specified, the default access is private.

A class can have only one level of inner classes. You can see this in the Force.com Apex Code Developer's Guide on page 56.

In Apex, you can define top-level classes (also called outer classes) as well as inner classes, that is, a class defined within another class. You can only have inner classes one level deep. For example:
public class myOuterClass {
// Additional myOuterClass code here
class myInnerClass {
// myInnerClass code here
}
}

You cannot have a private top-level class. It would not have any accessible functionality and would just be a waste of space. The default modifier for pretty much everything within a top-level Apex class, however is private. The only counter-example I can think of is getters/setters, which inherit from their property. Below, you can see that a getter without a specified level of access can still be public.

public String someProperty { get; private set; }

**A. A class can have multiple levels of inner classes.**
- **Explanation**: Apex supports nested inner classes, and these inner classes can themselves contain further nested inner classes, allowing multiple levels of nesting. This feature enables the encapsulation of functionality that is specific to the outer class, organizing code into a more manageable structure.

**D. Exception classes must end with the word exception.**
- **Explanation**: In Apex, custom exceptions must be defined by extending the built-in `Exception` class, and by convention (and for clarity and maintainability), the names of these classes should end with the word "Exception". This is a best practice that helps distinguish exception classes from other types of classes.