SQL can be used to:

Option A: create database structures only. - This option is incorrect. SQL (Structured Query Language) can do more than just create database structures. It can also query data and modify data.
Option B: query database data only. - This option is incorrect. SQL is not limited to querying data only. It can also create database structures and modify data.
Option C: modify database data only. - This option is incorrect. SQL is not limited to modifying data only. It can also create database structures and query data.
Option D: All of the above can be done by SQL. - This is the correct answer. SQL is a comprehensive language that can perform various tasks including creating database structures (DDL - Data Definition Language), querying database data (DML - Data Manipulation Language), and modifying database data.
Conclusion:
SQL can be used for Option D: All of the above can be done by SQL. This includes creating database structures, querying database data, and modifying database data. Therefore, the correct answer is All of the above can be done by SQL.