Step 1: Points and directions
Line 1 passes through \((1,-1,1)\) with direction \((2,3,4)\). Line 2 passes through \((2,-m,2)\) with direction \((1,2,1)\).
Join vector: \((1,1-m,1)\).
Step 2: Coplanarity determinant
\[ \begin{vmatrix}1&1-m&1\\2&3&4\\1&2&1\end{vmatrix}=0 \]
Step 3: Expand
\(1(3-8)-(1-m)(2-4)+1(4-3)=-5+2(1-m)+1=-2-2m\).
Setting it to \(0\) gives \(m=-1\). Option (C).
Final Answer:
\(m=-1\), option (C).
\[ \boxed{\text{(C) }-1} \]