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I'm not sure if I totally get this one. (A) says that "Most of the lead seals produced during the early byzantine Empire were affixed to documents that were then opened during that period."

We know that "most seals would have been recast" [reused] once the documents were opened. However, I suppose it would be possible that none of the documents were opened at all, and if this were true then the lead seals would not have been recast, and author could not conclude that the # of documents sealed with lead seals is many times the amount of lead seals that remain today.

I guess A sort of confirms this super subtle assumption, but can anyone else explain this question?

Yeah, according to the argument, the number of documents sealed with lead must be many times the 40,000 that have been found.

The author concludes this from the fact that the Byzantines melted down and reused seals that weren't used on certain important documents that weren't opened.

So what's the assumption?

Obviously, to argue that there were many more than 40,000 lead seals--on the basis of recycling that only occurred when documents were opened--assumes that most seals were affixed to documents that were opened. This is choice (A).