What does the possible_keys column in this output denote?

A MySQL Server has been running an existing application successfully for six months. The my.cnf is adjusted to contain this additional configuration:

The MySQL Server is restarted without error.
What effect will the new configuration have on existing account?

Consider:

Which statement best describes the meaning of the value for the key_lencolumn?

You have a MySQL instance with the following variables in the /etc/my.cnf file:

You issue these statements:
USE prices;
UPDATE sales.january SET amount=amount+1000;
An hour after excluding the statements, you realize that you made a mistake and you want to go to the binary log and look at the statements again.
Which statement is true? (Choose two.)

A crucial database, 'db_prod', just disappeared from your production MySQL instance.
In reviewing the available MySQL logs (General, Audit, or Slow) and your own application-level logs, you identified this command from a customer facing application:
SELECT id FROM users WHERE login='payback!';DROP DATABASE db_prod;'
Which three methods could have been used to prevent this SQL injection attack from happening? (Choose three.)