The correct answer is B) 25% of its breaking strain. The normal safe working load for used nylon rope in good condition is typically taken as 25% of its breaking strain. This is a commonly accepted industry standard based on guidelines and regulations for the safe use of ropes and rigging equipment. The 25% factor provides a reasonable margin of safety to account for potential wear, abrasion, and other factors that can reduce the rope's actual strength compared to its original breaking strain. The other options (10%, 33%, and 50%) are not the standard recommended safe working load for used nylon rope.