Explanation:
To calculate the buffer cache hit ration you need also to know physical reads. Formula for calculation is: 1- physical reads / (db_block gets + consistent gets).
Incorrect Answers:

2: Session logical reads statistic is not required to calculate the buffer cache hit ratio.
3: Table scan blocks gotten statistic is not used to calculate the buffer cache hit ratio.
4: DBWR buffer scans statistic is not required for this purpose also.
Oracle 8, DBA Certification Exam Guide, Jason S. Couchman, p. 859-863. Chapter 18: Tuning Memory and Operating System Use

---