What type of user-defined types can be included in Apex REST classes?

In Apex REST classes, user-defined types are expected to adhere to certain restrictions, particularly concerning the types of member variables they can contain. When creating an Apex REST service, the types that are utilized must include member variables of supported types. Supported types include standard data types such as strings, integers, booleans, collections, and other user-defined types that are also structured correctly.

This ensures that the data can be properly serialized and deserialized when requests are made to the REST service. Utilizing unsupported types, such as those that cannot be properly handled in JSON or XML formats, would lead to errors when the Apex REST service tries to process the incoming or outgoing payloads. Therefore, member variables of user-defined types must be compatible with the serialization mechanisms that Apex REST relies on.

In contrast, the other options highlight either incompatible attributes or suggest broader allowances that are not aligned with how Apex inherently manages data types for serialization in REST contexts. For instance, using types containing unsupported types, or claiming that any complex types are permissible, would result in serialization issues that could compromise the integrity of the REST service. Thus, the requirement for member variables of supported types is foundational for successful Apex REST class implementation.