Which feature is crucial for safety while operating heavy machinery like digger derricks?

The crucial feature for safety while operating heavy machinery, such as digger derricks, is the proper training of operators. Training ensures that operators are knowledgeable about the machinery's functions, safety protocols, and the potential hazards associated with its operation. Well-trained operators can effectively manage equipment, respond appropriately to emergencies, and adhere to safety guidelines, significantly reducing the risk of accidents and injuries.

The other choices, while relevant in some contexts, do not specifically address the primary safety concern. High-speed operation may increase risks and is not a safety feature. The use of basic tools is not particularly relevant to the safe operation of heavy machinery, as the complexity of such equipment often requires advanced tools and techniques. Additionally, the availability of spare parts is critical for maintenance and repair, but it does not impact the immediate safety of the operator during actual operation. Thus, proper training stands out as the key factor in ensuring safety.