Happy to help. This is a strengthen with a sufficient premise question. We're looking to add a premise that, if added to the stimulus, will make the argument valid.
(A) says, "Some societies have been geographically isolated enough not to have been influenced by any other society."
We know from the stimulus that all societies have money (we're told this in the first sentence). (A) states that some societies are isolated to the point where they are not influenced by other societies. If this is true, then the societies that are isolated (which also have money) had to have invented money on their own. This makes the conclusion true, so (A) is a sufficient premise that makes the argument valid, and it's the correct answer.
(D) says, "If money were not useful, it would not be so widespread."
We are not questioning the usefulness of money; rather, what we are looking to prove is that money was invented by different groups of people, including those societies that have always been isolated. Thus, (D)'s information adds nothing to the argument and is irrelevant, so we can get rid of it.
Does this make sense? Let us know if you have any other questions!
