Final answer:
Lake Tanganyika, stretching over 418 miles long along the border between the Congo and Tanzania, is the world's longest freshwater lake in the Great Rift Valley.
Step-by-step explanation:
The world's longest freshwater lake in the Great Rift Valley is Lake Tanganyika. This impressive lake measures over 418 miles long and is the entire border between the Congo and Tanzania. Besides its length, Lake Tanganyika is also remarkable for its depth, reaching approximately 4,800 feet, making it the world's second deepest lake. Furthermore, due to its volume, it is the second largest by volume after Lake Baikal in Russia. These features make Lake Tanganyika not only a geographical wonder but also an essential body of water supporting millions of people and a wide variety of wildlife.