(b) : Silicon belongs to group 14 , it is doped with a group 13 element like B which contains only three valence electrons. The place where the fourth valence $e^{-}$is missing is called electron hole. Under the electric field, $e^{-}$holes moves toward the negatively charged plate, which implies that $e^{-}$holes are positively charged, thus it is a $p$-type semiconductor.