Idiom: Blow The Whistle

As it’s world Cup time now, I’ve chosen an English idiom related to football: “Blow the whistle”.
The pronoun form is “whistle-blower”

What do you think it means?
“After working for the company for several years, his conscience made him blow the whistle on their illegal activities.”
a) Prevent it from happening again
b) Tell the authorities
c) Ignore it
d) Cover it up