d
Given system of equations is
$x + y + z = 6$
$x + 2y + 3z = 10$
$x + 2y + \lambda z = 0$
It has unique solution.
$\therefore \left| {\begin{array}{*{20}{c}}
1&1&1\\
1&2&3\\
1&2&\lambda
\end{array}} \right| \ne 0$
$ \Rightarrow 1\left( {2\lambda - 6} \right) - 1\left( {\lambda - 3} \right) + 1\left( {2 - 2} \right) \ne 0$
$ \Rightarrow 2\lambda - 6 - \lambda + 3 \ne 0 \Rightarrow \lambda - 3 \ne 0 \Rightarrow \lambda \ne 3$