Which of these is the probability mass function of a Poisson distribution?
e−λλkk!\frac{e^{-\lambda} \lambda^k}{k!}k!e−λλk
(nk)pk(1−p)n−k\binom{n}{k} p^k (1-p)^{n-k}(kn)pk(1−p)n−k
12πσ2e−(x−μ)2/2σ2\frac{1}{\sqrt{2\pi\sigma^2}} e^{-(x-\mu)^2/2\sigma^2}2πσ21e−(x−μ)2/2σ2
1b−a\frac{1}{b-a}b−a1