Which line handles the recursive case for this function that calculates the 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$.$

sum $=1$

b$.$

return sum

c$.$

if n $==0$

d$.$

sum $=$ n $+$ nsum$($n$-1)$

e$.$

elif n $==1$