$6-$It occurs when legitimate users are unable to access information systems, devices, or other network resources due to the actions of a malicious cyber threat actor. Services affected may include email, websites, online accounts $($e$.$g$.,$ banking$),$ or other services that rely on the affected computer or network. Find the missing word.

a$-$ Phshing

b$-$Nimidia

c$-$ Denial of Service

d$-$ SQL Slammer