Idiom: Move the goal posts

Here is another football-related idiom: move the goal posts

Our accountants had been working on several legal tax loopholes when the government suddenly moved the goal posts.

What does “moved the goal” posts mean?
a) The government sued us.
b) The government made new allowances that helped us.
c) The government changed the rules, making it more difficult.
d) The government investigated us.