Which one pair of parent out of the following is most likely to get a child , who would suffer from haemolytic disease of the new born?

Question

$\begin{array}{1 1}a.Rh^{+} \; mother \;and \;Rh^{-}\; father. \\ b.Rh^{-} \;mother\; and \;Rh^{-}\; father \\ c. Rh^{+} \;mother\; and\; Rh^{+}\; father \\ d.Rh^{-}\;mother\; and\; Rh^{+} \;father \end{array}$