Final answer:
Securities exchanges facilitate efficient markets by allowing supply and demand to determine securities prices and enable the allocation of funds to productive uses, but they do not control supply and demand through price.
Step-by-step explanation:
Securities exchanges play a crucial role in the functioning of financial markets by allowing for the trading of securities such as stocks. These exchanges provide a platform where market forces can determine the price of securities based on supply and demand. This facilitates the efficient allocation of funds, as investors can choose to invest in companies that they believe will be the most productive. Moreover, securities exchanges enforce a market where prices should, in theory, reflect the true value of a security with the caveat that this doesn't always occur in practice due to various factors such as information asymmetry or market manipulation.
Looking at the options provided in the question, option B is the one that securities exchanges do not do. They do not control the supply and demand for securities through price; rather, they provide a venue where supply and demand determine the price. Market prices are influenced by a myriad of factors, including investor sentiment, company performance, and broader economic indicators, but it is not the exchanges themselves that control these aspects. They merely facilitate trading based on these factors.