Final answer:
The event causing a decrease in supply of economics textbooks is the rising market price of paper. Increased demand or an increase in publishers would not decrease supply. In finance, a rise in demand or supply of funds can increase loan quantity, and a rise in supply can decrease interest rates.
Step-by-step explanation:
The event that would cause a decrease in the supply of economics textbooks is D. The market price of paper rises. When the cost of raw materials like paper increases, publishers may find it costlier to produce textbooks, which can lead to a reduced supply. Options A and B, where more students are buying books and economics textbooks become popular, would likely increase demand rather than reduce supply. Option C, an increase in the number of publishers, would presumably lead to an increased supply if anything. In the financial market, changes that lead to an increase in the quantity of loans made and received include a rise in the demand for loans and a rise in the supply of loanable funds. Similarly, a decline in interest rates can typically be attributed to a rise in supply of loanable funds, as more funds available usually means lenders have to compete by offering lower interest rates.