The LGWR process writes redo information from the redo log buffer to the online redo log files under a variety of circumstances: ■ When a user commits a transaction, even if this is the only transaction in the log buffer. ■ When the redo log buffer becomes one-third full. ■ When the buffer contains approximately 1MB of changed records. This total does not include deleted or inserted records. ■ When a database checkpoint is performed. ■ When 3 seconds have elapsed since the last redo log buffer write.

I believe both B and D are true. The LGWR background process does write to disk upon COMMIT: "When a user issues a COMMIT statement, LGWR puts a commit record in the redo log buffer and writes it to disk immediately, along with the transaction's redo entries. The corresponding changes to data blocks are deferred until it is more efficient to write them. This is called a fast commit mechanism." However, It also writes simultaneously to all multiplexed redo log files: "LGWR writes synchronously to the active mirrored group of redo log files." Reference : https://docs.oracle.com/cd/B19306_01/server.102/b14220/process.htm#i7261

When a user process commits a transaction – When an online redo log switch occurs – When the redo log buffer is one-third full or contains 1 MB of buffered data – Before a DBWn process writes modified buffers to disk

only D is correct