Question: Pumas never attack a human being except ... conered.

[Infin1ty]

Hello everyone,

I have a question to ask you. Here's a test:

Pumas never attack a human being except_____conered.
A) they are B) being C) that they are D) when they are
Which one is the best answer?

The reference answer is option D, but I think all these options are acceptable and make sense:

Pumas never attack a human being except they are conered.

Pumas never attack a human being except when they are conered.

Pumas never attack a human being except being conered.

Pumas never attack a human being except that they are conered.

Am I right? What do you think? I really failed to see why only option D fits the sentence while other options grammartically do not. Many thanks.

Please feel free to comment on my post if you find any mistakes in it, I'll really appreciate your kind help.
_________________
Today is a gift, that's why we call it the present.

[Kitosdad]

Good morning my friend. Only one correct answer in this example.

Pumas never attack a human being except they are cornered.

Pumas never attack a human being except when they are cornered.

Pumas never attack a human being except being cornered.

Pumas never attack a human being except that they are cornered.
_________________
If you need me, I'm here.

[Infin1ty]

It's 13:00 now in my area( GMT+8 hours), So, good afternoon, Bill. Thanks for your reply.
_________________
Today is a gift, that's why we call it the present.