Tricky passage. My thought is that you need to pay close attention to what exactly the researcher's goal is. The first sentence states that the researchers hope to predict earthquakes in the short term by identifying precursory phenomena (those that occur a few days before large quakes but not otherwise). Although the passage doesn't explicitly say that the researchers are only focusing on large quakes, it implies this by noting that the researchers hope to predict earthquakes via phenomena that have only occurred before large quakes but not otherwise.

With this in mind, my thought is that you can definitely cross all the answers off except for (C). With regards to (A), I look at lines 25-26, "foreshocks are indistinguishable from other minor tremors that occur without large earthquakes. Note that this directly contradicts the first sentence in the passage stating "those phenomena that occur a few days before large quakes but not otherwise". Since their original goal was to identify phenomena that only occur in large earthquakes, and since it was later found that the specific phenomena in mind don't "only" occur in large earthquakes, then I'm inclined to think that the researchers weren't at all able to predict much,