The probability generating function of a discrete non-negative integer valued random variable N is a function of the real variable t: $\displaystyle P(t)=\sum_{k=0}^{\infty}t^{k}*P[N=k]=E[t^N]$.

Which of the following is the correct expression for the probability generating function of the Poisson random variable with mean 2?

I have no idea of how to approach this. Any ideas?