The correct option is C 6543
Since the number of digits are 1,2 and 3 for first, second and third number respectively, so the next number in the series must have 4 digits.
Also, the next number in the series starts with the ending digit of previous number, so the upcoming number in the series must start with digit 6.
So, there is only one option which starts with 6 and has 4 digits, so option “c” will be the correct option.