Among the options provided, a) gold coins is the best example of commodity money. Commodity money refers to a type of currency that is made up of a commodity with intrinsic value. Gold coins have value not only as money but also due to the value of the gold itself. This is in contrast to other options like business stocks, which are financial instruments and not physical commodities; a valuable painting, which is an art object and not used as currency; and dollar bills, which are fiat money and do not have intrinsic value backed by a physical commodity.