The quiz asked "It's lame", "_____ nul".
The answer that it wants is "C'est nul".
Why can't it be "Il est nul"?
Suppose the sentence was a response to the question "What do you think of that film?"
The answer is providing an opinion with an adjective which it is applying to a specific thing - "that film".
That sounds a lot like case 2b in the lesson:
"2. Cases expressing opinions or simple statements (adjectives) about prementioned things"
"b. il est/elle est for statements and opinions related to specific things"