Mosasaurs were large marine reptiles that resembled a cross between a crocodile and a giant iguana. The mosasaurs are believed to have completely died out after a huge asteroid impact caused Earth to drastically change.

The question says the mosasaurs completely died out. When an entire species or group dies out, that is called extinction. So the correct choice is extinction; migration means moving to another place, and adaptation is a change that helps organisms survive.

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