The correct answer is option 4, tse-tse fly. The vector of the disease sleeping sickness is specifically the tse-tse fly. A vector is an organism that carries and transmits a disease from one host to another. In the case of sleeping sickness, the tse-tse fly becomes infected with the parasite that causes the disease when it feeds on the blood of an infected human or animal. The fly then transmits the parasite to another human or animal when it feeds again. It is important to note that options 1, 2, and 3 are incorrect. The sand-fly is the vector for diseases like leishmaniasis, the house-fly is a common vector for diseases like cholera, and the fruit-fly is not a vector for any known human diseases.