To determine which fossil would be the oldest based on the provided categories, we typically look at the evolutionary timeline of these organisms. Among corals, gastropods, and trilobites:
- Trilobites are considered some of the earliest known marine animals, existing from the Cambrian period around 521 million years ago to the end of the Permian around 252 million years ago.
- Corals have a more varied timeline, appearing later than trilobites, especially the modern corals developing in the Mesozoic era.
- Gastropods first appeared around the Cambrian period as well, but their significant diversification happened later.
Based on general knowledge about these groups, the trilobite would be the oldest fossil among the options listed.