Answer:
Option D. Change default user name and password.
Explanation:
Changing the default user name and password will make the network secure because only authorized persons will be able to access the network through the password (known only to them).
The default user name and password can be known even to unauthorized persons as it is displayed already on to the router.
So when the setting to change the password are done the network becomes secure as the password is known only to authorized persons and no one other than them could access it.
i hope it will help you!