What is an entity cluster, and what are the THREE ways one can be formed?

Answer

An entity cluster is a set of one or more entity types and their associated relationships grouped into a single abstract entity type to simplify the presentation of a large or complex E-R diagram; it can be formed by combining a supertype and all its subtypes, by combining directly related entity types and their relationships, or by combining a strong entity with its associated weak entities

It is a presentation/decomposition technique that hides detail behind an expandable abstract box — it never changes the underlying model and is never formed by deleting anything (sample exam Part A Q5 trap).