(D) Let the oxidation state of $Cr$ be $x$.In the complex $[Cr(NH_3)_6]Cl_3$,the oxidation state of $NH_3$ is $0$ and $Cl$ is $-1$.Applying the rule:

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(D) Let the oxidation state of $Cr$ be $x$.In the complex $[Cr(NH_3)_6]Cl_3$,the oxidation state of $NH_3$ is $0$ and $Cl$ is $-1$.Applying the rule: $x + 6 \times (0) + 3 \times (-1) = 0$.$x - 3 = 0$.$x = +3$.Therefore,the oxidation state of $Cr$ is $+3$.