Answer: A. December 31, 2018
Explanation: RMD also known as required minimum distribution is a withdrawal one has to take from his or her retirement plan once he or she attains the age of 70and a half years old.
According to IRAs, once a person attains the age of 70.5 which is six months after the person's 70th birthday, the individual is entitled to take his or her RMD by the 31st of December following his or her 70.5 birthday.
According to the above question, Walter is entitled to take his RMD on the 31st of Dec, 2018.