The correct option is D. competitive demand. Explanation of the Correct Answer Competitive Demand refers to the situation where two or more goods can satisfy the same need or want, making them substitutes for each other. When the demand for one good increases, consumers will tend to buy less of the other good because they are interchangeable in fulfilling the same purpose. In the case of butter and margarine, if the price of butter rises or if there is an increase in consumer preference for butter, more people will buy butter instead of margarine. This shift in demand means that the demand for margarine will decrease as consumers switch to the more preferred or cheaper option (butter). Why the Other Options Are Incorrect A. Composite Demand: This refers to a situation where a good is demanded for multiple uses. For example, if a commodity like steel is used in construction, manufacturing, and automotive industries, an increase in demand for steel for one of these uses does not necessarily reduce the demand for its other uses. Therefore, this option does not apply to the scenario of butter and margarine, which are substitutes rather than a single good with multiple uses. B. Elastic Demand: Elastic demand refers to a situation where the quantity demanded of a good is highly responsive to changes in price. For example, if the price of a luxury car increases, the quantity demanded may drop significantly. While butter and margarine may have elastic demand characteristics, the term does not specifically describe the relationship between two substitute goods. Thus, it does not fit the context of the question. C. Derived Demand: This type of demand occurs when the demand for one good is dependent on the demand for another good. For instance, the demand for labor is derived from the demand for the goods and services that labor helps produce. In the case of butter and margarine, the demand for one does not derive from the demand for the other; rather, they are directly competing substitutes. Therefore, this option is not applicable. Summary of Key Concepts

1. Competitive Demand: Goods that can replace each other; an increase in demand for one leads to a decrease in demand for the other.
2. Composite Demand: Demand for a good that has multiple uses; not applicable to substitute goods.
3. Elastic Demand: Demand that changes significantly with price changes; does not specifically relate to the relationship between substitutes.
4. Derived Demand: Demand that depends on the demand for another good; not relevant in the context of substitutes.

Revision Summary

• Competitive demand occurs when two goods can replace each other, leading to inverse demand relationships.
• Composite demand involves a single good used for multiple purposes, not applicable to substitutes.
• Elastic demand measures sensitivity to price changes, not the relationship between substitute goods.
• Derived demand is based on the demand for another good, which does not apply to the case of butter and margarine.

Understanding these concepts will help clarify the relationships between different types of demand in economics, especially in the context of substitutes.