How did Augustus Caesar establish the Roman empire's borders? O by using a system of tax collection
O by using a postal service
O by using natural boundaries
O by using merchant ships

He established the empire's boundaries along natural physical features. Troops were stationed along with the frontier areas of the Rhine River, Danube River, the Atlantic Ocean, the Sahara, and the Euphrates River to protect the empire from invaders.

Explanation:

So, by using natural boundaries

the branch of biology that deals with the normal functions of living organisms and their parts.

the way in which a living organism or bodily part functions.