As the length of the bit strings are n
And the strings are containing exactly r 1's
So, we have to choose r 1's from n-length string
the answer will be $^nC_r$ or $\dfrac{^nP_r}{r!}$→as all 1's are same
one question may arise that Why it is not a permutation problem?
Consider this string 11000, here n= 5 & r=2
now if we permute $^5 P_2$ then it would be 60
But there are two 1's & three 0's. So, $\dfrac{60}{{2}*{3}}$
And the generalized formula will be same as combination formula or the formula of permutation with repeating objects
Formula of combination ⇒ $^nC_r$ = $\dfrac{n!}{{(n-r)!}*{r!}}$
Formula of permutation with r repeating objects ⇒ $\dfrac{^nP_r}{r!}$ = $\dfrac{n!}{{(n-r)!}*{r!}}$