Answer: The right answer is B) Necessary.
Explanation: Just to elaborate a little on the answer, it can be added that although the word obligatory primarily means enforced or compulsory, I believe that in this excerpt it has been used with a hint of humor, as if suggesting that it was expected of the narrator to burn the draft card, and he did so, since everybody was doing it (in opposition to the Vietnam War). However, his main concern and occupation was his job at a printing press.