Explanation:
Constraints have functions to enforce rules at the view and table levels and to prevent the deletion of data if dependencies exist between tables.
Incorrect Answers:
1: Constraints are used to put dependencies on data.
2: Constrains are not used to make complex queries more easy.
6: Constraints does not prevent the deletion of an index if there are dependencies.
OCP Introduction to Oracle 9i: SQL Exam Guide, Jason Couchman, p. 227-248
Chapter 5: Creating Oracle Database Objects

---