The correct answer is $\boxed{-2}$.
To solve for the third root, we can use Vieta’s formulas. Vieta’s formulas state that the sum of the roots of a polynomial equation is equal to the negative of the coefficient of the $x^2$ term, and the product of the roots is equal to the constant term. In this case, the coefficient of the $x^2$ term is $-6$, and the constant term is $-6$. Therefore, the sum of the roots is $-6$, and the product of the roots is $-6$.
We know that two of the roots are $1$ and $3$. Therefore, the third root must be $-2$, since $1 + 3 – 2 = -6$ and $1 \cdot 3 \cdot -2 = -6$.
Here is a brief explanation of each option:
- Option A: $j$ is an imaginary number. The sum of the roots of a polynomial equation must be a real number, so $j$ cannot be the third root.
- Option B: $-j$ is also an imaginary number. The sum of the roots of a polynomial equation must be a real number, so $-j$ cannot be the third root.
- Option C: $2$ is a real number. The sum of the roots of a polynomial equation must be a real number, so $2$ cannot be the third root.
- Option D: $4$ is a real number. The sum of the roots of a polynomial equation must be a real number, so $4$ cannot be the third root.