In a binomial setting with $n$ independent Bernoulli trials, probability of success $p$, and failure probability $q=1-p$, the probability of exactly $r$ successes is

For binomial distribution, the number of ways to choose which $r$ trials are successes is ${}^nC_r$. Each such outcome has probability $p^r q^{n-r}$. So the required probability is ${}^nC_r p^r q^{n-r}$. Option B reverses success and failure exponents.