The vectors $2 \hat{\mathbf{i}}+3 \hat{\mathbf{j}}-2 \hat{\mathbf{k}}, \quad 5 \hat{\mathbf{i}}+a \hat{\mathbf{j}}+\hat{\mathbf{k}} \quad$ and
$-\hat{\mathbf{i}}+2 \hat{\mathbf{j}}+3 \hat{\mathbf{k}}$ are coplanar when ' $a$ ' is
(a) $-9$
(b) 9
(c) $-18$
(d) 18