Explanation:
Index will be useful if the indexed column contains a wide range of values. Especially B-tree indexes will work better for tables with a wide range of values. But for tables just with some distinct values bitmap indexes will be more helpful.
Incorrect Answers:
1: Oracle automatically creates index for NOT NULL columns, so you don’t need create an index yourself.
2: There are only table names in the FROM clause, not columns.
3: Index may be will not work if the indexed columns are part of an expression. You need to avoid expressions if you want to use index.
OCP Introduction to Oracle 9i: SQL Exam Guide, Jason Couchman, p. 322-331
Chapter 3: Creating Other Database Objects in Oracle

---