The Roman Empire added new territories mostly by:

O A. setting up trade routes with far-off countries.

B. attacking and conquering its neighbors.

O C. using its great wealth to buy territories.

O D. spreading its religious beliefs to foreign lands.

I want to say B, The Romans are a militaristic society but did not generally set out to conquer territory. But, got involved in many wars. After defeating enemies, Rome always offered them some level of citizenship in exchange for loyalty. So mostly by conquering its neighbors.

B. attacking and conquering it's neighbors.