probability – When do I use N for the variance of x-bar instead of n-1

Let X1, X2, X3 be random variables with the same variance σ^2.

Then the variance of x-bar= 1/3(X1+X2+X3) will necessarily be σ^2/3:

a. if the Xi’s are all normally distributed

b. always

c. if the Xi’s are independent of each other

d. if the Xi’s are identical distributed