Hi tizwvu,
The issue here is that an entirely new element appears in the conclusion (occupation of settlement at the time the engraving was done) that wasn't in any of the premises. This makes this question a Supporter Assumption one, in which we need to tie this new element in the conclusion to one of the premises (similar to a Justify approach). With that in mind, we can Prephrase to get a general idea of what the correct answer choice should look like: the engraving of the mammoth bone is assumed to a) have been done at the settlement, and b) done when the settlement was occupied, so that it couldn't have been brought in from elsewhere. (A) corresponds with the second Prephrase, so it's likely the correct answer choice; however, we can still test it with the Assumption Negation Technique:
The engraving wasn't made during the period the settlement was occupied
The engraving doesn't necessarily show that the settlement was occupied at a time when mammoths were in the area
Works like a charm, so this is definitely the correct answer.
Hope this clears things up!
