Firstly, the overall (addition of all) oxidation state of a neutral compound must be zero. In $[Pt(NH3)3Cl]Cl,$ the central metal atom is the Platinum(Pt).
Hence (d) is the correct answer.
Elements like H, F, Cl, O, N have its usual oxidation state as +1,-1,-1,-2,-3 respectively.
If we plug these oxidation states into the compound, $[x+3*(-3+3)-1]+(-1)$ will be the case.
Because this compound is a neutral compound, oxidation state for $Pt (x)$ should be $+2$.