The correct answer is A. Roman Legions.
The Roman Legions were the standing professional army of the Roman Empire. They were the primary military force of the Empire and were instrumental in expanding and defending its territory. The Roman Legions were made up of Roman citizens who were trained and equipped for battle, and they were considered to be among the most disciplined and effective fighting forces of their time.
The Roman Legions were able to spread Roman culture more than any other group because they were the main agents of Roman expansion and conquest. As they conquered new territories and established Roman rule, they also introduced Roman culture to the people they conquered. This included the Roman language, religion, legal system, and way of life, which were all imposed on the conquered peoples. Over time, these elements of Roman culture became prevalent throughout the Empire and influenced the culture of many other societies.
In contrast, the Roman Senate, Roman explorers, and Roman philosophers did not have as much influence on the spread of Roman culture. The Roman Senate was the governing body of the Roman Republic and later the Roman Empire, but it did not have the same military and territorial power as the Roman Legions. Roman explorers were responsible for exploring new lands and mapping the known world, but they did not have the same impact on the spread of Roman culture as the Roman Legions. Roman philosophers were influential in the development of Roman thought and culture, but they did not have the same level of influence as the Roman Legions on the spread of Roman culture to other peoples.