Reserve requirements are changed (LO2) a) once a week b) three or four times a year c) once every two or three years d) once every ten or fifteen years e) only if Congress passes a new law

First, it's important to understand what the reserve requirement is. It is the amount of cash a bank must have, in its vaults or at the closest Federal Reserve bank, in line with deposits made by customers. The requirement is set by the Federal Reserve, the central bank of the United States.

Now, consider the options in the given question. The question is asking how often these reserve requirements are changed. Your possible choices range from weekly changes to changes only happening if a new law is passed.

The answer to this question is based on how often the Federal Reserve decides to change these requirements. Typically, reserve requirements do not change frequently, and are not altered weekly, so option 'a' is not feasible. Also, changes are not as rare as every ten to fifteen years, so option 'd' can be excluded too. Changing reserve requirements only when a new law passes (option 'e') is also not the case because the Federal Reserve has freedom to adjust as they see fit economically. Then left are options 'b' and 'c', three or four times a year and once every two or three years. Comparatively, 'b' is too frequent while 'c' is closer to the actual practice by the Federal Reserve. Thus, the answer would be: c) once every two or three years

Thus, the frequency at which reserve requirements are changed is not very high due to the significant amounts of money and financial stability involved. It is a tool used sparingly by the Federal Reserve, with changes typically happening once every two or three years.