What did a brontosaurus eat?

The brontosaurus was a herbivore, living on plants only. It was not a meat eater.

Based on the structure of its teeth, it probably browsed on leaves.

Note: an extensive study published in 2015 by a joint British-Portuguese research team concluded that Brontosauruswas a valid genus of sauropod distinct from Apatosaurus, but not all paleontologists agree on that yet.