Problems encountered during instrumentation :: (A) Ledging., (B) Stripping / lateral wall Perforation ., (C) Zipping., (D) All of the above.
Answer is D. Problems Encountered during Instrumentation

Loss of working length due to a blockage of the canal system
Ledging (shelf formation)
Breakage of instrument
Deviation from the normal canal anatomy
Zipping (elliptication)
Stripping/lateral wall perforation
Inadequate canal preparation
Overinstrumentation (beyond apical constriction)
Over preparation (excessive removal of tooth structure)
Under preparation (insufficient canal preparation)