Thanks for the question! Let’s take a look at the stimulus first. We’re told that ancient humans in eastern North America hunted mammoths until they disappeared. Then, we’re told a fossil bone was found with a mammoth picture on it at some ancient settlement in the area. The author concludes that the settlement was occupied when the mammoths were still around. That’s all the last sentence is telling us.
Now the question asks us for an assumption. This is a question that we can potentially prephrase. What are things that argument assumes? Well, one thing it assumes is that people wouldn’t have made a fossil of mammoths after they went extinct, or before they existed. It also assumes that the engraving was actually made at the settlement and not brought there from somewhere else, and that it was done when the settlement was occupied (as opposed to brought in from elsewhere). This is what (A) tells us, and so (A) is the correct answer.
Let’s try negating this assumption to see if it’s required for the argument. Let’s say that the engraving wasn’t made during the time when the settlement was occupied. Then it’s not the case that the engraving necessarily shows the settlement was occupied when mammoths were around.
Hope this helps. Feel free to ask any further questions that you might have.