"Nemluvte o věcech, o nichž nic nevíte."

Translation: Don't talk about things you know nothing about.

[dsarkarati]

"Don't talk about things WHICH you know nothing about." is the same as "Don't talk about things THAT you know nothing about." which is the same as "Don't talk about things you know nothing about." All three options are correct.

[thomasmarsh]

I had "Don't talk about things which you know nothing about". It should be accepted