which period is older than the triassic period?\no cambrian\no cretaceous\no jurassic\no paleogene

Brief Explanations:

The Triassic period is part of the Mesozoic Era. The Cambrian period is from the Paleozoic Era and predates the Mesozoic. The Cretaceous and Jurassic are also from the Mesozoic and are younger than the Triassic. The Paleogene is from the Cenozoic Era and is younger.

Answer:

A. Cambrian