Which of the following is not a river in Africa?

indus

a. Niger

b. Tigris

c. Zambezi

d. Congo

Niger and Congo are two rivers after which countries are named (actually in both cases two countries each) - and those are African countries, so they must be in Africa.

Zambezi is a river in Angola and Namibia.

Tigris is a River in Mesopotamia, in Asia - so that's the correct answer!