Explanation:
Since EMPLOYEE_ID column is used as primary key, it cannot be NULL, so only INSERT statements in C, E and F are correct. You can insert the row with NULL LAST_NAME as in answer C, or only the row with EMPLOYEE_ID as in answer E, or the row with empty LAST_NAME column.
Incorrect Answers:
1: This answer is incorrect because a primary key cannot be NULL.
2: INSERT statement does not contain primary key value at all, so this answer needs to be eliminated as correct one.
4: This statement shows incorrect order of columns of row which needs to be inserted into the table.
OCP Introduction to Oracle 9i: SQL Exam Guide, Jason Couchman, p. 260-267
Chapter 6: Manipulating Oracle Data

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