The persons who saved the lives of the passengers are c. Both Semyon and Vasily helped to save the lives of the train passengers.
Who save the lives of the passengers?
Semyon helped to save the lives of the passengers by soaking the scarf in his blood to make a red flag. In the process of doing this, he almost lost his own life. Although Vasily was the main person who caused the accident, he also tried to prevent further loss of lives.
So, the third sentence clearly states that both Semyon and Vasily were involved in saving the lives of the train passengers and this option is correct.