Which two statements are correct about the trigger to be created for the above requirement?

A and C
check here :https://docs.oracle.com/cd/B28359_01/server.111/b28318/triggers.htm#CNCPT1677

a , d

http://www.dba-oracle.com/t_avoiding_mutating_table_error.htm

If we create a row level trigger, it will fire for every single row that is getting deleted. Instead we should create statement level trigger which will fire once the entire statement is executed.

Each new release of the Oracle database reduces the impact of the mutating table error on triggers and they are much less of a problem with Oracle9i and above. If a trigger does result in a mutating table error, the only real option is to rewrite the trigger as a statement-level trigger.

Mutating table errors only impact row level triggers. But to use a statement level trigger, some data may need to be preserved from each row, to be used by the statement level trigger. This data can be stored in a PL/SQL collection or in a temporary table. A simple row level trigger that causes a mutating table error can result in a very complicated statement level trigger to achieve the needed result.

Here are some important items to remember about triggers.

On insert triggers have no :OLD values.

On delete triggers have no :NEW values.

Triggers do not commit transactions. If a transaction is rolled back, the data changed by the trigger is also rolled back.

Commits, rollbacks and save points are not allowed in the trigger body. A commit/rollback affects the entire transaction, it is all or none.

Unhandled exceptions in the trigger will cause a rollback of the entire transaction, not just the trigger.

If more than one trigger is defined on an event, the order in which they fire is not defined. If the triggers must fire in order, you must create one trigger that executes all the actions in the required order.

A trigger can cause other events to execute triggers.
A trigger can not change a table that it has read from. This is the mutating table error issue.

The fact that a trigger can cause other triggers to fire is an important item to remember. A trigger that causes other database events to execute triggers can cause the database crash. For example, the database can capture server errors by defining a trigger on the database server error event.

But if this trigger causes a server error, the database will spin in a loop, with each firing of the trigger causing the error, firing the trigger again, and again, and again. The only way to regain control of the database is to disable the trigger.

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