Correct Answer: (a) Washington D.C.
Solution:The Fourth Nuclear Security Summit, 2016 was held from 31st March, 2016 to 1st April 2016 in Washington D.C., the capital of the United States of America. A total of 56 world leaders from 52 countries and four international organizations participated in this summit. It is noteworthy that Russia decided not to attend the Nuclear Security Summit, 2016. After, 2016 there is no information available about NSS 2018, 2021. However the main aim of the NSS is the secure all vulnerable nuclear materials.