Answer: b. Because of unpredictable changes in the public's desire to hold cash or borrow and banks' desires to hold reserves or lend.
Step-by-step explanation:
The Fed is able to embark on monetary policy that influences the entire country - and the world to some extent - because they have very strong influence over the money supply of the US$.
This influence is not absolute however because as the old adage goes, "you can lead a horse to water but you can't make him drink". In other words, the Fed can relax(impose) restrictions to make money more(less) available but they cannot force people to borrow(hold) that money.
They can't force banks either to either hold reserves or lend money out because banks are free to impose their own reserve limits on top of those of the Fed.