Monetarists accept the idea that velocity is not constant, but they believe that it is well-behaved, predictable, and dependent on the money supply. Therefore, the correct option is b. unpredictable, well-behaved, and dependent on money supply.
Monetarists believe that changes in the money supply affect changes in nominal GDP in a predictable way, and that velocity is a key factor in this relationship. They argue that as long as the money supply is stable and predictable, velocity will be stable and predictable as well, and changes in nominal GDP can be accurately forecasted. However, they also acknowledge that velocity can vary in the short run due to shifts in consumer and investor behavior, changes in technology, and other factors, which can make it unpredictable in the short run.