It can't be done; it's far too difficult / It shouldn't be done; it's far too difficult

[kroshka-kartoshka]

In Passive Quiz https://www.usingenglish.com/quizzes/227.html in question №7 choosing option 3 is considered an error -
It ....... done; it's far too difficult
1) must be
2) shouldn't be
3) can't be
Instead it offers option 2. But doesn't it just a semantic difference? Option 3 is about the impossibility of realisation because of complexity, while option 2 is about known meaninglessness of all attempts. Could both of these options be correct in a passive context?

 

[Piscean]

In Passive Quiz https://www.usingenglish.com/quizzes/227.html in question №7 choosing option 3 is considered an error -
It ....... done; it's far too difficult
1) must be
2) shouldn't be
3) can't be
Instead it offers option 2. But doesn't it just a semantic difference? Option 3 is about the impossibility of realisation because of complexity, while option 2 is about known meaninglessness of all attempts. Could both of these options be correct in a passive context?

Yes.
I would choose option 3.

 

[jutfrank]

It's an obvious editing mistake. The correct answer should be option 3. The other two don't fit the context.

 

[Tarheel]

In Passive Quiz https://www.usingenglish.com/quizzes/227.html in question №7 choosing option 3 is considered an error -
It ....... done; it's far too difficult
1) must be
2) shouldn't be
3) can't be
Instead it offers option 2. But ISN'T it just a semantic difference? Option 3 is about the impossibility of realisation because of complexity, while option 2 is about known meaninglessness of all attempts. Could both of these options be correct in a passive context?

If somebody says something can't be done they mean it's impossible. Only #3 works there.