Concept:Effective supply refers to the actual quantity of a good that producers are willing and able to sell in the market at a given price.Explanation:Effective supply is not the same as total production or total stock in a warehouse.It is the amount of a commodity that is genuinely available and offered for sale to buyers at a particular market place.Only goods that are actually put on sale in the market can form part of effective supply.Therefore, it excludes goods that are merely produced, kept in storage, or supplied by a single producer.Answer:C. offered for sale at a market place