Comoros is the only country in the Arab world that is entirely in the Southern Hemisphere. Located in the Indian Ocean off the eastern coast of Africa, Comoros is an archipelago consisting of three main islands: Grande Comore, Mohéli, and Anjouan. Let's explore the reasons why Comoros is the correct answer to this question.

Geographical Location:
Comoros is situated in the southern part of the Arabian Peninsula, near the equator. It lies between latitudes 11° and 13° south and longitudes 43° and 45° east. Being entirely south of the equator, it is the only Arab country that falls in the Southern Hemisphere.

Surrounded by Water:
Comoros is surrounded by the Indian Ocean on all sides. It is located off the eastern coast of Africa, northeast of Mozambique and northwest of Madagascar. The country's location in the Southern Hemisphere is further emphasized by its position in the Indian Ocean, which predominantly lies south of the equator.

Arab World:
The Arab world consists of 22 countries, primarily in the Middle East and North Africa. While most of these countries are located in the Northern Hemisphere, Comoros is an exception. Its unique geographical location makes it the only Arab country that is entirely situated in the Southern Hemisphere.

Other Options:
Mozambique, Djibouti, and Mauritania are not entirely in the Southern Hemisphere. Mozambique is located in both the Northern and Southern Hemispheres, with the majority of its landmass lying south of the equator. Djibouti is mainly in the Northern Hemisphere, with a small portion extending into the Southern Hemisphere. Mauritania is entirely in the Northern Hemisphere, situated in northwest Africa.

In conclusion, Comoros is the only country in the Arab world that is entirely in the Southern Hemisphere. Its geographical location in the Indian Ocean, south of the equator, distinguishes it from other Arab countries.