for Embeddable classes, it is not required to implement Serializble.
That's correct. But there is a second option to extract fields into a separate class other than using
embeddable classes: Just take a serializable class as type of the field (the persistence provider
then stores the serialized object in the db). And this second option is given by a).
Of course, one can't say that Address class must implement Serializable (it can also be taken to be
a non-serializable embeddable class). But similar one can't say that Address class must be
embeddable (it can also be a serializable, non-embeddable class).