To add a new column to an existing table, the DDL command used isAALTER TABLEBMODIFY TABLECCHANGE TABLE

The DDL command used to add a new column to an existing table is A) ALTER TABLE.

Here is a step by step guide on how to do it:

1. First, you need to know the name of the table to which you want to add the new column. Let's assume the table name is 'students'.

2. Decide on the name and data type of the new column. Let's say we want to add a new column named 'age' and the data type is 'int'.

3. Now, you can use the ALTER TABLE command to add the new column. The syntax is as follows:

   ALTER TABLE table_name ADD column_name column_type;

4. So, in our case, the command will be:

   ALTER TABLE students ADD age int;

5. Run this command in your SQL environment to add the new column to the 'students' table.

Remember, you need to have the necessary privileges to alter a table. If you don't, you'll need to ask your database administrator to grant you the necessary permissions.