The underlined sentence usually contains one grammar pattern doing the real work, something like a connector (-아서/어서, -는데, -지만) or an ending that carries a specific nuance (-어야 하다, -을 수도 있다). The rest of the sentence in each of the four choices stays almost identical; only that one grammar point gets swapped out for something that looks similar. Only one swap actually preserves the original meaning. The other three usually shift something small hiding inside the new grammar: the timing, the certainty, or the cause-and-effect relationship.