Critical Reasoning

Excavations of the Roman city of Sepphoris have uncovered numerous detailed mosaics depicting several readily identifiable animal species: a hare, a partridge, and various Mediterranean fish. Oddly, most of the species represented did not live the Sepphoris region when these mosaics were created. Since identical motifs appear in mosaics found in other Roman cities, however, the mosaics of Sepphoris were very likely created by traveling artisans from some other part of the Roman Empire

Which of the following is an assumption on which the argument depends?

A)The Sepphoris mosaics are not composed exclusively of types of stones found naturally in the Sepphoris area.

B) There is no single region to which all the species depicted in the Sepphoris mosaics are native

C) No motifs appear in the Sepphoris mosaics that do not also appear in the mosaics of some other Roman city.

D) All of the animal figures in the Sepphoris mosaics are readily identifiable as representations of known species

E) There was no common repertory or mosaic designs with which artisans who lived in various parts of the Roman Empire were familiar

This is QID 2302 in the GMATFix Solutions Engine (full solution here).

An assumption is something that must be true for the conclusion to follow from the premises.

Premises:
animals depicted on motifs did not live in the city.
Identical motifs appear in other Roman cities

Conclusion:
traveling artists from other parts of the empire created the motifs found in our city.

Make a prediction:
Do we know this conclusion to be true for sure? How could it be wrong?

The conclusion would make no sense if:
1) local artists from our city traveled to other cities and copied motifs they found elsewhere
2) motifs were painted elsewhere and just sent to our city.

If either of any of statements were true, the conclusion would make no sense. so the author assumes that these statements are false.

Answer E corresponds to (1) above. it's an assumption. Note that negating E (saying that "there is a common source from which artists from all over paint the same designs") destroys the conclusion.

Answer E