Explanation:
This statement provides correct syntax to add a primary key on the STUDENT_ID column of the STUDENT table.
Incorrect Answers:
1: This ALTER TABLE statement is missing CONSTRAINT keyword and the name of the constraint.
2: This ALTER TABLE statement is missing the name of the constraint.
3: It’s incorrect syntax in the ALTER TABLE command: STUDENT_ID must be used with brackets.
5: We need to add constraint, not to modify existing one. Usage of the MODIFY keyword is incorrect in this case.
OCP Introduction to Oracle 9i: SQL Exam Guide, Jason Couchman, p. 239-240
Chapter 5: Creating Oracle Database Objects

---