Final answer:
Julius Caesar was famous for conquering large sections of Europe, particularly during his war against the Celts of Gaul and his invasion of Britain. Hence, the correct answer is option (a).
Step-by-step explanation:
Julius Caesar was famous for conquering large sections of Europe. During his campaigns from 58 to 50 BCE, Caesar waged a brutal war against the Celts of Gaul, which is present-day France. His military successes extended his influence and the territory of the Roman Republic significantly.
An interesting fact is that Caesar became the first Roman general to invade Britain, establishing a Roman presence there. The regions that were most affected by his conquests were in Europe. Caesar's legacy includes the thorough conquest of the lands he invaded, to the extent that the descendants of the Celts in these regions eventually spoke languages based on Latin, such as French.