Final answer:
Casey Jones is the engineer who died with one hand on the brake lever during an unavoidable crash on April 30, 1900, saving the lives of his passengers.
Step-by-step explanation:
On April 30, 1900, the engineer who died with one hand on the brake lever in an unavoidable crash was Casey Jones (a). He was known for his heroic act of slowing down the train to minimize the impact, which saved the lives of all the passengers at the cost of his own. This incident highlights the significant improvements in railway safety that had been made since the invention of the air brake by George Westinghouse in 1869, which allowed trains to stop more effectively and thereby travel at higher speeds safely. It marks a moment in history where human bravery intertwined with technological advancements to avert a greater disaster.