It is easy to see how laypeople could consider firefighting to involve risk that is freely assumed because becoming a firefighter is a voluntary career choice and the risks involved are well-known. Therefore, if you choose to become a firefighter, it's pretty safe to say that you are freely assuming the risk inherently involved in fighting fires.
It's a bit tougher to understand why travelling in airplanes is viewed as involuntary as people generally choose to travel by air of their own volition.
Answer choice (D) is correct because of the discussion from lines 25 to 32. The author tells us that the risks incurred by airline passengers are typically thought to be involuntary because passengers have no control over whether a plane is going to crash. They also tell us that people tend to focus on the accident as opposed to the decision to fly.
As the author is outlining what is "typically thought" about travelling in airplanes, we can conclude that laypeople generally consider that the risks assumed by airplane passengers are not freely assumed.
Hope this helps! Please let us know if you have any further questions.
