The statement that is not a function that money serves is:
"ensures that the double coincidence of wants is satisfied in a barter system."
In a barter system, trade occurs directly through the exchange of goods and services without the use of money, which is why the double coincidence of wants (where each party must want what the other has) is a requirement. Money, on the other hand, eliminates the need for this double coincidence by acting as an intermediary in transactions.