Eh. All of these COULD indeed work, depending on what you're trying to communicate. A is the "most correct" one, as that's the one that would be used in most situations and is simply stating a fact that someone would, indeed, think such a thing. B is odd, but if you were demanding that a person should think something (like if you were a Jedi and you were mind-tricking some dude), it could work. C is also odd, but if you were giving a person permission to think something (like they're some sort of mindslave and you have to give them orders to have thoughts), it could work. D is the next best answer. It's a sentence that would be followed up with a clarification that the person is wrong. "You might think they'd be here by now, but then you'd be making bad assumptions." Because the typical meaning of this sentence is conveyed with "would", that's the answer I'd choose if I was forced to do so. But this sentence is so vague that, given the specific circumstances, even B and C might be okay. However, because the circumstances that would make B and C right are purely fictional ones, and because D is a condescending sort of sentence that somebody would almost never say, my choice is A.