Here's how I approached it:

First, a two second summary of archaeologist claim: Because porcelain cups have unique compositions, archaeologists can use composition analysis to determine where the next discovered cup comes from.

So looking at the statements, I try to eliminate the statements that do not support the archaeologist's claim:
a) RELEVANT: Says that they have previous porcelain cups, so these are the cups that they could use for their composition analysis
b) ELIMINATE: Cups that are not made of porcelain do not support his claim
c) RELEVANT: The next cup has to be porcelain, archaeologist only talks about identifying cups that are porcelain
d) ELIMINATE: Hurts archaeologist claim because it introduces reasoning to why composition analysis might not lead to correct origin of cup
e) ELIMINATE: Hurts archaeologist claim because composition analysis might not point to correct origin if two origins produce the same type of composition
f) ELIMINATE: Seems like irrelevant information

So between A and C, it's not too hard to tell which one is the archaeologist assumptions and which one is possible fact. The archaeologist is assuming that they can find out where the next cup comes from because the next cup will be porcelain, and they can use composition analysis. The fact that all of the cups they have previously found were porcelain strengthens the assumption that the next one will be porcelain as well.

If you reversed it, and said that the assumption is all of the past cups found were porcelain, the possible fact that the next one will be porcelain does nothing to strengthen your assumption.