Quote: Editorialist: Despite the importance it seems to have

A universal loss of belief in it will make money disappear.
We witness this phenomenon on a small scale daily in the rises and falls of financial markets.

Conclusion: Money does not exist.

We are concluding that money does not exist by saying that if people stop believing in it, it will disappear. So we are assuming that if something disappears because of lost beliefs, it doesn't really exist. We are assuming that anything that exists should continue to exist even if people stop believing in it. If it disappears when people stop believing in it, it doesn't exist.

We need a sufficient assumption.

A) Anything that exists would continue to exist even if everyone were to stop believing in it.

Correct. As discussed above, we are assuming that anything that exists would continue to exist even if people did not believe in it.

B) Only if one can have mistaken beliefs about a thing does that thing exist, strictly speaking.

We don't know whether beliefs are mistaken or not hence this is irrelevant.

C) In order to exist, an entity must have practical consequences for those who believe in it.

We don't know what is meant by the term practical consequences. Irrelevant.

D) If everyone believes in something, then that thing exists.

No. The argument says that if everyone stops believing in something and that makes the thing disappear, then the thing doesn't really exist in the first place. The argument says that money doesn't exist even though people believe in it. It says that this because if tomorrow people stop believing in it, it will disappear. Our argument talks about what happens when people stop believing in a thing. So a statement that tells us what happens when people believe in something is irrelevant.

E) Whatever is true of money is true of financial markets generally.

We would have given it a thought if it were reversed - whatever is true of financial markets, is true of money. But as given, this is irrelevant.

Answer (A)