Test analysis: Commemorative coins are also legal tender issued by the state, which can be used for commodity exchange. As a medium of commodity exchange, they have the function of circulation means, so D is correct; The essence of commemorative coins is a symbol of value, so A is wrong; Money is issued by the state, but its purchasing power is not stipulated by the state, so B is wrong; The collection value of commemorative coins is determined by many factors, so C is wrong. The answer to this question is D.