C. a memory aid
A mnemonic is a memory aid, usually a phrase or visual, that helps you remember a fact.
My car is my main way of transportation.
Duty, in a way, contributed to the shallowness and failure of Ivan's life. Not because duty in itself is bad, but because it has become Ivan's only preoccupation, apart from pleasant life. Tolstoy's negative view of duty is seen in the second half of this sentence: "he considered his duty all things that were so designated by people in authority". So, we are talking about duty as Ivan sees it, and Ivan sees it as it was designated by people in authority. That means, Ivan structures his life and his priorities according to the view of those above him. His only ambition is to try and climb the social ladder. When he fails to do it, he is disappointed and has to find another obsession - of course, materialistic one. And that is the decoration of his new house. That's when he falls down and injures himself - so, indirectly, his materialism has cost him his health and, ultimately, his life.
