The correct options are
B 3π/4
C π/4
The diagonals are given by AB−BC=4i−2j+4k,AB+BC=2i−2j
These vectors have magnitudes 6 and 2√2 respectively, and their dot product is 12.
Therefore the angle between them is cos−112(6)(2√2)=cos−11√2=π4
or π minus this value i.e.3π/4