Answer: d. iodine and calcium
Explanation: Ionic bonds are formed by transfer of electrons between metal and non metals.
Covalent bonds are formed by sharing of electrons between non metals
Electronic configuration of calcium:
[tex][Ca]=[Ar]4s^2[/tex]
Calcium atom will lose two electron to gain noble gas configuration and form calcium cation with +2 charge.
[tex][Ca^{2+}]={Ar][/tex]
Electronic configuration of iodine:
[tex][I]=[Kr]4d^{10}5s^25p^5[/tex]
Iodine atom will gain one electron to gain noble gas configuration and form iodide ion with -1 charge.
[tex][I^-]=[Kr]4d^{10}5s^25p^6[/tex]
In calcium iodide the one electron from calcium metal gets transferred to iodine atom and thus form an ionic bond to give [tex]CaI_2[/tex]