Answer:
Option C
Explanation:

Vector perpendicular to face OAB= →n1
= →OA×→OB
= (ˆi−2ˆj+ˆk)×(−2ˆi+ˆj+ˆk)
=(−2−1)ˆi−(−2−1)ˆj+(1−4)ˆk)
= −3ˆi−3ˆj−3ˆk
Vector perpendicular to face ABC= →n2.
= →AB×→AC
= (−3ˆi+3ˆj)×(ˆj+ˆk)
= −3ˆi+3ˆj−3ˆk
Since, angle between faces is equal to angle between their normals.
∴ cosθ=→n1.→n2|→n1||→n2|
=(−3)(3)+(−3)(3)+(−3)(−3)√9+9+9√9+9+9
=−9−9+9√27√27=−13
⇒θ=cos−1(−13)