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Your example is listed under Just Eat Gilligan as a subversion, I think specifically because it was brought up by the characters. Normally the trope is for quick and easy solutions that no character ever mentions but seems obvious to the viewers, like "Why don't we have the Giant Eagles fly the Ring to Mordor instead of traveling on foot?" (Apparently this question was brought up in the book, but omitted in the film.)
There is also Stating the Simple Solution, although that seems to be geared more toward overly complicated villain plans when they would be much better served by just shooting someone with an ordinary gun.
Edited by KD
I think it would just be subverted Just Eat Gilligan. Wonder if there's enough examples for it to be a trope.
^ That might actually be a valid trope, come to think of it.
Currently Working On: Incorruptible Pure Pureness
There's like 9 subversions on that page. I'd say that's enough for a TLP.
The best example I have of this is in LOTR with Tom Bombadil.
The council of Elrond brings him up as someone who is not affected by the ring- someone who would potentially be useful in taking it.
The response is:
"That won't work because..."
Is there a name for this sort of bringing up questions that readers are likely asking just to explain why it won't work?