Final answer:
The situation described, where the price of coffee has increased but the quantity traded in the market has decreased, would be caused by a decrease in demand. Therefore, the correct answer is C. Decrease in demand.
Step-by-step explanation:
The situation described, where the price of coffee has increased but the quantity traded in the market has decreased, would be caused by a decrease in demand. According to the law of demand, when the price of a good increases, the quantity demanded decreases. Additionally, the decrease in quantity traded indicates a decrease in supply. Therefore, the correct answer is C. Decrease in demand.