The oxidation number of Pt in the complex ${\left[\mathrm{Pt}\left({\mathrm{C}}_2{\mathrm{H}}_4\right){\mathrm{Cl}}_3\right]}^{-}$ is

Assume that x represents the Pt oxidation state in ${\left[\mathrm{Pt}\left({\mathrm{C}}_2{\mathrm{H}}_4\right){\mathrm{Cl}}_3\right]}^{-}$ 
The complex's overall charge is -1, hence the total sum of all the components' oxidation states should also be -1. 
Consequently, X+0 -3 =-1 

X=+2 

Therefore the correct option is B +2