A relational database is a type of database that organizes data into tables, where each table has a fixed number of columns and a variable number of rows. Each row in a table represents a record or an entity, and each column represents an attribute or a property of that entity. The tables are linked by common fields, called keys, which enable the database to establish relationships between the data. A relational database schema is a diagram that shows the structure and organization of the tables, columns, keys, and constraints in a relational database. The diagram given in the question isan example of a relational database schema, as it shows two tables: ''Runs'' and ''Experiments'', with their respective columns, data types, and primary keys. The ''Runs'' table also has a foreign key that references the ''ExperimentId'' column in the ''Experiments'' table, indicating a relationship between the two tables. Therefore, the correct answer is D. Reference: What is a database schema? | IBM, Database Schema - Javatpoint