If $O(A)=2\times 3$, $O(B)=3\times 2$ and $O(C)=3\times 3$, which one of the following is not defined?

Check definitions:$O(B)=3\times 2⟹O(B^{\prime })=2\times 3$.$A+B^{\prime }$ is defined because both are $2\times 3$. Resulting order is $2\times 3$.For $C(A+B^{\prime })$ to be defined, the columns of $C$ must match the rows of $(A+B^{\prime })$.$O(C)=3\times 3$, $O(A+B^{\prime })=2\times 3$.Columns of $C=3$; Rows of $(A+B^{\prime })=2$.$3\ne 2$, so $C(A+B^{\prime })$ is not defined.