In economics, all of the following are included among the three main uses of money except
a.
store of value
b.
weight of gold
c.
unit of account
d.
medium of exchange

B. weight of gold

Explanation:

Money is a term used to describe an item that is generally accepted  in transactions involving  transfer of goods from a person to another.  Money is that item accepted as the medium of exchange. Money used in a country is that country's currency. Each nation has its currency.  

Money has three main functions which are a) medium of exchange, b) store of value, and c) Unit of account.

Store of value means that the money must maintain its worth over a reasonable duration of time. The strength attached to money should not be fluctuating. If one dollar can buy ten units of an item today, then the same dollar should be able to purchase around ten items one year later.

As unit of account,  other goods and services value can be expressed in monetary form. For example, the cost of a car is ten thousand dollars.

The weight of gold is measured per ounce. It can be given a monetary value, but it's not a function of money.  In ancient trading, gold was used in place of money.

i think the answer to your question is b. directive