Explanation

(d)
The formula of given complex are as follows:
(a) Hexammineplatinum (IV) chloride
$\left\lbrack Pt\left( {NH}_{3} \right)_{6} \right\rbrack{Cl}_{4}$
(b) Chloropentammine platinum (IV) chloride
$\left\lbrack Pt\left( {NH}_{3} \right)_{5}Cl \right\rbrack{Cl}_{3}$
(c) Dichhlorotetrammine platinum (IV) chloride
$\left\lbrack Pt\left( {NH}_{3} \right)_{4}{Cl}_{2} \right\rbrack{Cl}_{2}$
(d) Trichlorotriammine platinum (IV) chloride
$\lbrack Pt\left( {NH}_{3} \right)_{3}{Cl}_{3}\rbrack Cl$
In aqueous solution the complex ionise is
$\lbrack Pt\left( {NH}_{3} \right)_{3}\ {Cl}_{3}\rbrack Cl \rightleftharpoons \lbrack Pt\left( {NH}_{3}{Cl}_{3} \right\rbrack^{+} + \ Cl$
Trichlorotriammine platinum (IV) 2 ions chloride gives the minimum number of ions in the solution. Hence, it has the minimum electrical conductivity.