世界で一番長い川は何ですか。

English Translation

What is the longest river in the world?

河, also pronounced as かわ, is interchangeable with 川 in this context.

河 derived from a Chinese expression mainly refers to long/wide rivers in Japanese whereas 川 can include short rivers and streams.
河川(かせん)covers any kinds of rivers and streams. 河川 in On Yomi (音読み) sounds more formal than かわ in Kun Yomi (訓読み), so academic and formal writings prefer 河川.