Lời giải:

\( mp(R) \perp mp(P):\) \( \vec{n}_P = (1, -1, 1) // \text{mp}(R). \)  
 \( mp(R) \perp mp(Q):\) \( \vec{n}_Q = (3, 2, -12) //\text{mp}(R). \)  

\( \vec{n}_R = [\vec{n}_P, \vec{n}_Q] = (10, 15, 5) // (2, 3, 1). \)  

\( \Rightarrow\) Phương trình mp(R): \( 2x + 3y + z = 0 \Rightarrow \boxed{C}\)