ANS:B - an oblique plane

Cast iron is very strong in compression but weak in shear. When a cast iron specimen is subjected to a compression test, the perpendicular cross-section bears the direct compressive stress. But a plane inclined at 45 degree to the normal plane has shear stresses due to resolved compression force.

Thus this plane is subjected to shear stress. As said earlier, cast iron is weak in shear. So failure occurs along this inclined or oblique plane.