Iran is the only country with coastlines on both the Caspian Sea and the Persian Gulf.
Iran’s northern coast borders the Caspian Sea, the world’s largest inland body of water, while its southern coastline reaches the Persian Gulf and the Gulf of Oman. This gives Iran a distinctive position connecting the Caucasus and Central Asia with the Arabian Peninsula and the wider Indian Ocean region.
The Caspian coastline lies along Iran’s northern provinces, including Gilan, Mazandaran and Golestan. To the south, Iran faces the Persian Gulf across a coastline that includes the strategically important Strait of Hormuz. The strait links the Persian Gulf with the Gulf of Oman and is one of the world’s most important energy-shipping passages.
The other options border only one of the two waters: Turkmenistan and Azerbaijan reach the Caspian, while Iraq reaches the Persian Gulf. The Caspian is often called a sea, but geographically it is an endorheic lake with no natural outlet to the world’s oceans. Iran’s two-water geography is therefore a strong clue in this question.