In a supertype/subtype hierarchy within EER modeling, which statement best reflects the standard structural rule for subtype membership?

Introduction / Context:
Supertypes and subtypes capture generalization and specialization in conceptual data modeling. The hierarchy organizes common properties at the supertype and specialized properties at subtypes. The question asks about the common structural rule.

Given Data / Assumptions:

• We are referring to a typical single-inheritance EER hierarchy.
• Subtypes refine the supertype with extra attributes or relationships.
• Multiplicity between supertype and subtype is many subtypes to one supertype in membership, not attributes.

Concept / Approach:
The standard rule in a classic hierarchy is that each subtype has exactly one supertype (single inheritance). While some modeling approaches allow multiple inheritance, hierarchy typically implies a tree structure, not a directed acyclic graph with multiple parents.

Step-by-Step Solution:

Verification / Alternative check:
Modeling tools and textbooks depict subtype triangles or arcs feeding into a single supertype, emphasizing one parent per subtype in a hierarchy.

Why Other Options Are Wrong:

• Only one attribute for a subtype or supertype is unrelated to hierarchy rules.
• Each supertype having only one subtype is false; a supertype generally has multiple subtypes or sometimes none.

Common Pitfalls:
Confusing hierarchy with multiple inheritance models; the word hierarchy is a strong indicator of single parentage.

Final Answer:
Each subtype has only one supertype.