The answer would be letter D: A sense of accomplishment mixed with a feeling of regret.
While growing up people live their life based on different decisions made through it. People take risks and drove their life the way they want but at the same time there are lots of risks people don't take. That is a resume of how life goes. Based on this, in the older years people start to realize all they accomplished through life, where they are standing and how much effort that took, but at the same time, with all of these accomplishment older people tend to think "What if", based in all the decisions or risks they never took. This brings in them a mixed sense of regret of things they never did and accomplishment of those they did.