1)Most of the species represented did not live in the Sepphoris region when these mosaics were created.
+
2)Since identical motifs appear in mosaics found in other Roman cities
==> 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?

What could be an assumption here? An assumption is something on which the arguments depends on, for example "the mosaics were not created in one place and then transported in all the other cities". This is an assumption that if negated destroys the argument.

That assumption targets the CONCLUSION alone, another type of assumptions is the one that undermines a connection premise/conclusion; example
1)===>conclusion.
An assumption here would be something like "the species were not caught in another region and then transported in the other cities".

a. The Sepphoris mosaics are not composed exclusively of types of stones found naturaly in the Sepphoris area.
The materials are nowhere mentioned in the passage.
b. There is no single region to which all the species depicted in the Sepphoris mosaics are native.
Even if this is negated, the argument still holds true, as this assumption does not target a specific part of the argument.
c. No motifs appear in the Sepphoris mosaics that do not also appear in the mosaics of some other Roman city.
Negate C: "some motifs appear in the Sepphoris mosaics that do not also appear in the mosaics of some other Roman city", so some motifs are unique=> the conclusion is still valid.
d. All of the animal figures in the Sepphoris mosaics are readily identifiable as representations of known species.
Negate D: "All of the animal figures in the Sepphoris mosaics are NOT readily identifiable as representations of known species"; even if they are not are NOT readily identifiable, they could still have been made by traveling artists.
e. There was not a common repertory of mosaic designs with which artisans who lived in various parts of the Roman Empire were familiar.
This is the assumption needed. If you negate it:
"There was not a common repertory (...)"=> the conclusion is destroyed "the mosaics of Sepphoris were very likely created by traveling artisans".
If there were such repertory, the local artists could have made such mosaics.

Fixed