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This is the problem with social conventions.

I eliminated every answer choice but A and got it right.

However, I would like a better technique. I just feel like if I insert that assumption into the argument, I would not be able to validly conclude that it is "PROBABLE" that the invention of money occurred independently in more than one society.

Bump for this frustrating question for me. How can that assumption make that conclusion probable? I know it says SEEM probable, but that is probable nonetheless correct?

Money is universal, or maybe nearly universal (premise).
Money is not innate, it must be developed (premise).
Some cultures developed in perfect isolation (assumption).

Therefore, money has probably independently developed several times. How else do these isolated cultures get money?

There are only two possibilities, money was developed once and it spread to every other money using culture, which is almost every culture; or it developed multiple times. The latter seems more likely than the former given the premises and our commonsense knowledge of the world.