Answer:
Carthage's conquering of Saguntum, which was a Roman ally.
Step-by-step explanation:
The Second Punic War was fought between Carthage and Rome primarily over the capturing of Saguntum by Hannibal, a Carthaginian leader. The town of Saguntum became the main bone of contention between the two powers which ultimately led to the Second Punic War.
Rome and Carthage had been in a constant war for territorial expansion. And with the Romans succeeding in the First Punic War, Carthage began looking for any chance to defeat the Romans. And in conquering Saguntum, a Roman ally, Carthage made a huge mistake which only led Rome to further attack Carthage.
Thus, the correct answer is the first option.