a. Every ball has n choices. Hence .
b. By the hint, arrange n balls and b-1 sticks in a row. The begin to the first stick is in the first bin, and so on. So there are ways.
c. By the hint, arrange n identical balls and b-1 sticks in a row. So there are latex {b \choose n}$ ways.
e. We select b balls for every bin. So there are n-b balls that should be placed in b bins. There are ways.