d
(d) The system of equations has infinitely many (non-trivial) solution,
if $\Delta = 0$ i.e., if $\left| {\,\begin{array}{*{20}{c}}3&{ - 2}&1\\\lambda &{ - 14}&{15}\\1&2&{ - 3}\end{array}\,} \right| = 0$
==> $3(42 - 30) - \lambda (6 - 2) + 1( - 30 + 14) = 0$
==> $\lambda = 5$ .