One of these is not a plant fossil

The correct option is Archaeopteryx.

To understand why this is the correct answer, let's look at each of the choices provided and determine whether they are plant fossils or not:
1. Rhynia: This is a genus of early Devonian vascular plants. Fossilized specimens of Rhynia gwynne-vaughanii represent a classic example of an early land plant, making it a well-known plant fossil.
2. Lepidocarpon: This is a genus of fossilized lycopsid seed-like megasporangia (reproductive structures) from the Carboniferous period, which belong to the plant kingdom.
3. Lepidodendron: Also known as the "scale tree," this is an extinct genus of primitive, vascular, arborescent (tree-like) plants related to lycopods, prominent during the Carboniferous period.
4. Archaeopteryx: Unlike the others, Archaeopteryx is a genus of avian dinosaurs. It is a famous transitional fossil representing the evolutionary link between non-avian theropod dinosaurs and modern birds. Therefore, it is an animal fossil, not a plant fossil.

Consequently, Archaeopteryx is the only organism listed that is not a plant fossil.