Introduction:
SQL (Structured Query Language) is a standard programming language used for managing relational databases. It provides various commands to perform different operations on a database. In this question, we need to identify which of the given options is not an SQL command.

Explanation:
Let's go through each option to determine whether it is an SQL command or not.

a) CREATE:
The CREATE command is used to create a new table, view, index, or other database objects in SQL. It allows us to define the structure and characteristics of the object being created.

b) DROP:
The DROP command is used to delete an existing table, view, index, or other database objects in SQL. It permanently removes the specified object from the database.

c) ALTER:
The ALTER command is used to modify the structure of an existing database object in SQL. It allows us to add, modify, or delete columns, constraints, or other properties of a table, view, or index.

d) All are SQL commands:
This option suggests that all the given options (CREATE, DROP, and ALTER) are SQL commands. However, this is incorrect because the ALTER command is indeed an SQL command. Therefore, option 'D' is not the correct answer.

Correct answer:
The correct answer is option 'D' because all the given options (CREATE, DROP, and ALTER) are SQL commands, and the question asks for the option that is not an SQL command.

Conclusion:
In SQL, the CREATE, DROP, and ALTER commands are used to create, delete, and modify database objects, respectively. The option 'D' stating that all the given options are SQL commands is incorrect.