Final answer:
The constraint directly expressed in the schemas of a data model is the Explicit constraint, ensuring integrity and consistency within the database.
Step-by-step explanation:
The constraint expressed directly in the schemas of a data model is C. Explicit. These are the constraints that are defined within the database structure itself and not outside of it. They include constraints on data types, uniqueness, primary and foreign keys, and not null constraints among others. These constraints ensure the integrity and consistency of the data within the database. For example, a primary key constraint ensures that each row in a table is unique, and a not null constraint ensures that a column cannot have a NULL value.