What phisical feature served as the most northern border of the Safavid Empire?

The physical feature that served as the northernmost border of the Safavid Empire is the Caucasus Mountains.

Explanation:

The Caucasus Mountains is a very wide and tall mountain range that strands from the Black Sea to the Caspian Sea, and that marks the border between Eastern Europe and a part of Asia (more specifically: the Middle East).

The Caucasus Mountains were the northemost border of the Safavid Empire, probably because the Mountains were too steep for the Safavids to pass, or because they served as an strategic defensive position.

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explanation: