To be used as a primary key, which of these characteristics must an attribute have in a single instance of the entity? Select all that apply. There must be a maximum of one attribute instance. There must be many instances of the attribute. There must be at most one entity instance that each attribute instance describes. There must be at least one instance of the attribute. There must be zero or more instances of the attribute.

To determine the characteristics that an attribute must have to be used as a primary key in a single instance of an entity, let's analyze each option: 1. **There must be a maximum of one attribute instance.** - This is true for a primary key. A primary key must uniquely identify a single instance of an entity, meaning there cannot be multiple instances of the primary key for the same entity. 2. **There must be many instances of the attribute.** - This is not true. A primary key should uniquely identify each instance of an entity, so it should not have many instances. Instead, it should have exactly one instance per entity. 3. **There must be at most one entity instance that each attribute instance describes.** - This is true. A primary key must uniquely identify one entity instance, meaning that each value of the primary key can only correspond to one instance of the entity. 4. **There must be at least one instance of the attribute.** - This is true. For an attribute to serve as a primary key, it must have at least one instance to identify an entity. 5. **There must be zero or more instances of the attribute.** - This is not true. A primary key cannot have zero instances; it must have at least one instance to identify an entity. ### Summary of Correct Characteristics: - There must be a maximum of one attribute instance. - There must be at most one entity instance that each attribute instance describes. - There must be at least one instance of the attribute. Thus, the correct characteristics for an attribute to be used as a primary key are: - There must be a maximum of one attribute instance. - There must be at most one entity instance that each attribute instance describes. - There must be at least one instance of the attribute.