The phrase "a preposterous proposition" is correct and usable in written English.
It can be used to describe an idea or suggestion that is absurd or ridiculous.
Example: "The idea that we could solve world hunger overnight is a preposterous proposition."
Alternatives: "an absurd suggestion" or "a ridiculous idea."
That is a preposterous proposition.
But Disney's adaptation of Edgar Rice Burroughs's John Carter of Mars books is such a preposterous proposition – the opener to a mega-budget franchise, directed by a first-time live-action film-maker, set on a Mars that doesn't remotely resemble the real red planet and based on a series of books from a weird outdated genre which hardly anyone has read – that I can't help rooting for it.
And just having spent so much time with Nick, it just seemed sort of a preposterous proposition to us, to try to take away that stuff because it felt to us like all of that mythology and all of that creation was a part of who Nick really is".
At first glance this looks like a pretty preposterous proposition: Planet Hulk is one of those outlandish sci-fi-style storylines beloved of Marvel that might not translate well to a live-action big-screen movie, where screenplays need to be built around the charisma of the actors involved rather than pseudo-mystical subplots and mysterious extraterrestrials.
That's an inherently preposterous proposition".
Putting aside the former President's preposterous proposition and despite all the concern over rising temperatures, even Bill Clinton's heated rhetoric won't spare us from a more likely threat: abnormally cold weather.
What a ludicrous proposition.
It's a ludicrous proposition".
Back then I explored this group and some of its more (preposterous) propositions: there should be an uprising against "The New World Order", of which the US government was a puppet; there was a site in Nevada where the militias believed a crashed UFO had been taken, the government plotting with aliens.
"It is an absurd proposition".
