A trigger is

A trigger is a special type of database statement or procedure.

It is executed automatically when a specific event occurs in the database.

For example, a trigger may execute automatically when:

A record is inserted

A record is updated

A record is deleted

So, a trigger is not manually executed by the user every time.

It is automatically executed by the DBMS as a side effect of modification to the database.

Now check the options:

Option (a) is incorrect because trigger is not used to start DBMS.

Option (b) is incorrect because trigger is not mainly used for debugging.

Option (c) is incorrect because user validity checking is related to authentication or authorization.

Option (d) is correct because trigger executes automatically after database modification.