An iodide ion may combine with one $I_2$ or two $I_2$ molecules to form $I_3^{-}$ and $I_5^{-}$ ions. Regarding these ions the correct statements is 

$I_3^{-}$ linear the central iodine atom is involved in $s{p}^3$ hybridization with three lone pairs occupying the three equatorial positons of trigonal bipyramid structure. During the formation of $I_5^{-}$ from $I_3^{-}$, one on the terminal iodine atom donate a lone pair to $I_2$ molecule and involves in $s{p}^3$ hybridization with two lone pairs and two bond pairs so it is angular.