Explanation:
After an incomplete recovery, always issue ALTER DATABASE OPEN RESET LOGS Incorrect answers:
1: ALTER DATABASE OPEN would not clear the redo logs and besides, it is synonymous with another of the choices (choice D; ALTER DATABASE OPEN NO RESET LOGS. ALTER DATABASE OPEN, alone, implies "NO RESET LOGS")
2: I do not believe that this is a valid command (I do not believe it exists in Oracle)
4: Same reason that choice A is incorrect. (ALTER DATABASE OPEN = ALTER DATABASE OPEN NO RESET LOGS)

---