Keeping $20 in currency to be able to buy gasoline, money is performing which function? unit of account symbol of fiat medium of exchange barter mechanism store of value

Store of value

Store of value is a general term used to refer to that function of an asset that can be saved in the present time, and retrieved and exchanged in the future. Common examples of this include money, currency, precious metals or financial capital. The store of value function of money means that the present purchasing power of money can be retained into the future; thus, a person can decide to save money (such as keeping the $20 in the example) to be able to use it sometime in the future (to buy the gasoline later as in the example). 

The answer should be b.