Swahili is the major Bantu lingua franca of East Africa.
Also called Kiswahili, Swahili developed along the East African coast among Bantu-speaking communities. Centuries of trade across the Indian Ocean brought sustained contact with Arabic-speaking merchants and settlers, giving Swahili substantial Arabic vocabulary while its grammar and core structure remained Bantu. The name itself comes from an Arabic word associated with the coast.
Swahili spread inland through commerce, religious teaching, migration and colonial administration. It became especially important as a second language, allowing people with different mother tongues to communicate across Tanzania, Kenya, Uganda, Rwanda, the Democratic Republic of the Congo and neighbouring areas. Tanzania and Kenya helped establish standardized forms written in the Latin alphabet.
The alternatives are also Bantu languages, which makes the question tempting: Shona is strongly associated with Zimbabwe, Xhosa with southern Africa, and Zulu with South Africa. They are important regional languages, but Swahili is the one traditionally identified as East Africa’s broad lingua franca.