Question

what natural barrier formed the southern border of the roman empire? the mediterranean sea the alps mountain range the black sea the sahara desert

Explanation:

The Roman Empire's southern border was formed by the Sahara Desert. The Mediterranean Sea was to the north (for some regions), the Alps to the north (in Europe), and the Black Sea to the northeast. The Sahara acted as a natural barrier to the south.

Answer:

The Sahara Desert