Although artists in different countries developed their own approach to realism, what common interest united them?

The correct answer should be a desire to present an unflinching look at the lives of the working poor.

Realism was all about reality, and portraying life as it actually is. Realist artists wanted to show life in its true light, painting the working, the poor, the scary, the tragic, all that wouldn't be usually shown in works of art during previous eras. They didn't want to hide anything, but rather show what life is actually like.