Answer: (the last option) An omen suggesting a tragic end to the battle
Step-by-step explanation:
This sort of imagery is used by Shakespeare when the natural order of things is about to be disturbed. The storm means Caesar's death is not appreciated by the gods and will certainly bring tragedy with it. Cassius, however, upon seeing the storm, chooses to interpret it as a sign that the gods do not want Caesar to be crowned an emperor. His choice of misinterpretation leads to his own ruin. Ironically enough, it was a misinterpretation that made him kill himself. Someone wrongly reports that Cassius' army has been defeated, which makes him kill himself.