The matrix multiplication of a square/non-square matrix depends upon the order of the matrix.
Let $A_{m*n}$ and $B_{n*p}$ are two matrix then the order of resultant $[AB]_{m*p}$
now,
$x_{1*3}=\begin{bmatrix}3&1&2 \end{bmatrix}$
$x^T_{3*1}=\begin{bmatrix} 3\\ 1 \\ 2 \end{bmatrix}$
- form option A, [$x^Tx]_{3*3}$ is true.
- option B is false because size of $xx^T$ should be $1*1$
- option C is true.
- option D is false.
$\text{option A,C is correct.}$