Once every how many miles, one of those mistakes leads to a near-collision?

Answer: D — 500

Small errors by any driver are a regular part of driving, and safety training emphasizes that you should expect and prepare for them so you can avoid a crash or near-collision. The idea here is that a near-miss caused by a common mistake tends to occur with a practical frequency across trips, and about every 500 miles fits that typical expectation. So 500 miles is the best match among the options because it reflects how often small lapses can accumulate into a dangerous moment on average. To stay safer, maintain a healthy following distance, scan ahead and to the sides, use mirrors and check blind spots, and minimize distractions so you can spot and react to mistakes by others before they become real hazards.