Which condition is the recursive case for sum of the first n natural numbers?def nsum $($n$)$:if n $==0$:sum $=0$elif n $==1$:sum $=1$else:sum $=$ n $+$ nsum $($n$-1)$return sum a$)$ if n $==0$ b$)$ elif n $==1$ c$)$ sum $=1$ d$)$ sum $=$ n $+$ nsum$($n$-1)$