In 1980, a Danish ten-øre coin minted in 1747 was sold at auction for

1980 - a coin was sold for 8k (past)
EB owns another such coin (present, 2020). It will fetch her 18k next week (one week from now).

We need to weaken that the coin will fetch 18k next week.

(A) Since 1980, the average price for rare coins has increased by over 150 percent.

Since 1980 to now, the price has increased by over 150%. So then it makes sense that the coin will cost 18k.

(B) There are only four coins like the one in question in the entire world.

Makes it likely that the coin will fetch a lot of money since there are only 4 such coins.

(C) Since 1980, the consumer price index has risen by over 150 percent.

Again, this means that the cost of things has increased by over 150%. So likely that the coin will fetch more than 18k.

(D) In 1986, a previously unknown cache of one hundred coins just like the one in question was found.

In 1986, many such coins were found. So it means now such coins are not rare. Then it is likely that it will not fetch a lot of money. Hence it weakens our conclusion.

(E) Thirty prominent, wealthy coin collectors are expected to bid for Bixby’s coin.

Irrelevant

Answer (D)