Answer
Iodine will Gain 1 electron
Barium will Lose 2 electrons
Cesium will Lose 1 electron
Selenium will Gain 2 electrons.
Work Step by Step
Since $Iodine$ is in group 7 and a non-metal, it will gain one electron to obtain a full octet so it becomes $I^{-}$.
Since $Barium$ is in group 2 (and a metal), it will lose two electrons to obtain a full octet so it becomes $Ba^{2+}$.
Since $Cesium$ is in group 1 and a metal, it will lose one electron to become stable so it becomes $Cs^{+}$.
Since $Selenium$ is in group 6 and a non-metal, it will gain two electrons to become stable so it becomes $Se^{2-}$.
