The pair of compounds that can not exist together is
(i) FeCl₃, SnCl₂ 
(ii) HgCl₂, SnCl₂
(iii) FeCl₂, SnCl₂ 
(iii) FeCl₃, KI

(i) FeCl_{3 ,} SnCl₂
 Fe⁺³  + Sn⁺²  $\to$  Fe⁺² +Sn⁺⁴
Oxidant    reductant
because FeCl₂ & SnCl₂ react with each other they cannot exist together
 

(ii) HgCl₂, SnCl₂
   Hg⁺² + Sn⁺²   $\to$ Hg⁺ + Sn⁺⁴

oxidant      reductant

because HgCl₂ & SnCl₂ react with each other they cannot exist together
 

(iii) FeCl₂, SnCl₂
Fe⁺² + Sn⁺² $\to$ no reaction
 

(iv) FeCl₃ , KI
   Fe⁺³ +   I^-  $\to$ Fe⁺² + I₂
Oxidant    reductant
 because Fe⁺³ & I^- react with each other they cannot exist together
therefore I,II,IV  cannot exist