Final answer:
The preposition that completes the sentence is "on".
Step-by-step explanation:
The preposition that completes the sentence is on. The sentence should be "Over thousands of people perished on the earth."
This sentence suggests a catastrophic event where a large number of people died, possibly due to a natural disaster, epidemic, or war.
Examples of usage:
- Over thousands of people perished on the earth as a result of the earthquake.
- Over thousands of people perished on the earth during the war.
This preposition indicates that the people died on the surface of the planet, as opposed to under, above, or inside it.