http://docs.oracle.com/cd/E11882_01/server.112/e25494/dbrm.htm#ADMIN11844

If a program unit does not need to be executed with the escalated privileges of the definer, you should specify that the program unit executes with the privileges of the caller, also known as the invoker. Invoker's rights can mitigate the risk of SQL injection.
Incorrect Answers:
A: By default, stored procedures and SQL methods execute with the privileges of their owner, not their current user. Such definer-rights subprograms are bound to the schema in which they reside.
not B: Using the GRANT option, a user can grant an Object privilege to another user or to PUBLIC.

Explanation/Reference:
Explanation:
AB: You can make individual table columns invisible. Any generic access of a table does not show the invisible columns in the table. For example, the following operations do not display invisible columns in the output:
* SELECT * FROM statements in SQL
* DESCRIBE commands in SQL*Plus
* %ROWTYPE attribute declarations in PL/SQL
* Describes in Oracle Call Interface (OCI)
Incorrect Answers:
D: You can make invisible columns visible.
You can make a column invisible during table creation or when you add a column to a table, and you can later alter the table to make the same column visible.

Explanation/Reference:
Explanation:
D: DB_FILE_MULTIBLOCK_READ_COUNT is one of the parameters you can use to minimize I/O during table scans. It specifies the maximum number of blocks read in one I/O operation during a sequential scan.
The total number of I/Os needed to perform a full table scan depends on such factors as the size of the table, the multiblock read count, and whether parallel execution is being utilized for the operation.