Schema consistency: do all instances (occurrences) of a given entity class share the same set of attributes (though values may be null or optional)?

Introduction / Context:
In ER and relational modeling, an entity class (type) defines a fixed schema: a set of attributes and constraints that every instance obeys. The question asks whether each instance of the same entity class has the same attributes.

Given Data / Assumptions:

• We are dealing with classic relational/ER modeling, not schemaless NoSQL.
• Optional attributes may be null for some instances.
• Subtype/supertype hierarchies may split attributes across related entity types, but each specific entity type still has a fixed attribute set.

Concept / Approach:
Yes—the definition of an entity class/type includes its attributes. Every instance conforms to that schema. Optionality affects whether a value is present, not whether the attribute exists. In relational terms, all rows of a table have the same columns; nullability governs missing values.

Step-by-Step Solution:
Define entity class/type: a template specifying attributes and constraints.Observe that instances are created according to that template; therefore, they share the attribute set.Account for optionality: attributes may be null, but they still exist for each instance.Therefore, the statement is correct.

Verification / Alternative check:
In SQL, CREATE TABLE defines columns (attributes); every row has those columns. Subtypes may be separate tables or single-table inheritance, yet each concrete type has a defined set of attributes.

Why Other Options Are Wrong:
Claiming invalidity ignores schema semantics; tying correctness to subtyping or storage engines confuses logical design with implementation; insisting on “no optional attributes” conflates presence of values with presence of attributes.

Common Pitfalls:
Assuming optional attributes are absent; misunderstanding inheritance—each subtype still has a defined schema.

Final Answer:
Valid statement