References:https://docs.oracle.com/cd/B28359_01/server.111/b28320/initparams240.htm#
REFRN10214

Not A: Both indexes are created fine.
B: The invisible index ORD_CUSTOMERS_IX1 and the bitmap index are both updated by DML operations on the Orders table.
F: Since ORD_CUSTOMERS_IX1 is invisible only ORD_CUSTOMERS_IX2 is used by the query optimizer.
Not C, Not D, Not E:
* ord_customer_ix1 is an invisible index and is therefore not used by the optimizer.
* VISIBLE | INVISIBLE Use this clause to specify whether the index is visible or invisible to the optimizer. An invisible index is maintained by DML operations, but it is not be used by the optimizer during queries unless you explicitly set the parameter OPTIMIZER_USE_INVISIBLE_INDEXES to TRUE at the session or system level.
Note: Specify BITMAP to indicate that index is to be created with a bitmap for each distinct key, rather than indexing each row separately. Bitmap indexes store the rowids associated with a key value as a bitmap. Each bit in the bitmap corresponds to a possible rowid. If the bit is set, then it means that the row with the corresponding rowid contains the key value.
The internal representation of bitmaps is best suited for applications with low levels of concurrent transactions, such as data warehousing.

Explanation/Reference:
References: https://docs.oracle.com/cd/E11882_01/server.112/e22490/dp_overview.htm#SUTIL2880

References:
http://docs.oracle.com/cd/B19306_01/server.102/b14220/process.htm (see log writer process (LGWR))