a: Wrong. The first thing that you have to notice is that the sum goes to infinity. If you want k=4 to be the last condition, then take out the 3 dots.
b: That's the answer.
c: wrong. You get a real mess when you let set k = 0. Try it on your calculator.
1 ÷ 0 = Watch carefully as your calculator mentally melts down.
d: wrong. It's just not right. The highest power is k^2. There is no way to get k^3
