To protect its frontiers, the roman empire depended on mercenaries. what is a mercenary?

a soldier who fights only for pay
a treaty with a neighboring territory
a military division that patrols borders
a neighboring tribe

Mercenary is a soldier who fights only for pay

Thus the correct answer is Option A.

Explanation:

Mercenaries were soldiers who would attack anyone if they were pain a ransom. Rome is said to be the most ancient and vast empire which had a great military war strategy. Over expansion militarily was the reason for its downfall too.  

From Atlantic Ocean to Euphrates River, Rome expanded the borders but in order to protect itself from the attacks of barbaric tribe, it had to rely upon mercenaries who were robust and sturdy soldiers who would attack anyone if they were paid. The only disadvantage of them was that they never paid allegiance to Rome. Whoever paid them, they fought for them.  

They were inexperienced too and not well trained. Rome had common subjects too in the army but they were not well trained these factors led to the fall of Rome.

protected civil liberties and outlawed slavery in the new territories.