The correct answer is 161009. This date most likely refers to the debut of the K-pop group B2ST (also known as Beast). The numbers in the answer represent the date in YYMMDD format, with 16 indicating the year 2016, 10 indicating the month of October, and 09 indicating the 9th day of the month. Therefore, October 9, 2016, is the most likely date for B2ST's first debut.