Series to be classifiable as a weakly stationary process

Which of the following conditions are necessary for a series to be classifiable as a weakly stationary process?
(i) It must have a constant mean
(ii) It must have a constant variance
(iii) It must have constant autocovariances for given lags
(iv) It must have a constant probability distribution
a)
(ii) and (iv) only
b)
(i) and (iii) only
c) (i), (ii), and (iii) only
d) (i), (ii), (iii), and (iv)