Which of the following is not a river in africa?
a. niger
b. tigris
c. zambezi
d. congo

Niger and Congo are both rivers in Africa - in fact two countries were called after each of these rivers.

Zambezi is also an African river - it flows through many countries, such as Angola and Zambia.

Tigris however (together with the River next to it, the Euphrates is close to Africa but it's actually in Asia - so this is the correct answer.