Answer:

The correct answer is option 'A': DDL (Data Definition Language).

Explanation:

DDL (Data Definition Language) is a type of SQL (Structured Query Language) command that is used to define the structure and organization of a database. It is responsible for creating, altering, and dropping database objects such as tables, views, indexes, and schemas. The command provided by DDL to define relation schema is called CREATE TABLE.

Relation Schema:
A relation schema defines the structure and organization of a table in a database. It specifies the name of the table, along with the names and data types of its attributes (columns). It also defines the primary key, foreign keys, and constraints associated with the table.

Example:

Let's consider an example to understand how DDL is used to define a relation schema.

Suppose we want to create a table called "Employees" with the following attributes:
- EmployeeID (integer)
- FirstName (string)
- LastName (string)
- Age (integer)
- DepartmentID (integer)

The DDL command to define this relation schema would be:

```
CREATE TABLE Employees (
EmployeeID INT,
FirstName VARCHAR(50),
LastName VARCHAR(50),
Age INT,
DepartmentID INT
);
```

In this example, the CREATE TABLE command is used to define the relation schema for the "Employees" table. It specifies the names and data types of the attributes, as well as the table name.

Other SQL Commands:

- DML (Data Manipulation Language): DML commands are used to manipulate data within the tables. Examples of DML commands include SELECT, INSERT, UPDATE, and DELETE.

- TCL (Transaction Control Language): TCL commands are used to manage transactions in a database. Examples of TCL commands include COMMIT, ROLLBACK, and SAVEPOINT.

- DQL (Data Query Language): DQL commands are used to retrieve data from the database. The most common DQL command is SELECT, which is used to query the database and retrieve specific data based on specified criteria.

While DML, TCL, and DQL are important components of SQL, they do not provide a command for defining relation schema. That is why the correct answer is DDL.