In the following question, select the missing number from the given series.
3, 7, 21, 56, 308, 1, ?

Note: The series printed as "3721563081?" is a single sequence of distinct multi-digit terms or individual digits parsed carefully under competitive exams formatting. Let's look closely at the segment relationships:
3×7=21
Next segment: Let's observe the numbers 56 and 308. If we take pairs or a consecutive product logic:
Let's see if it's a product pattern or digit-by-digit logic:
Given the standard layout of this question from SSC archives:
The string is split as: 3,7,21,56,308,… or it follows a pattern of alternate items.
Let's re-examine the question text as individual digits: 3,7,2,1,5,6,3,0,8,1,?
Sum of pairs:
3+7=10
2+1=3
5+6=11
3+0=3
8+1=9

Let's check product of pairs:
3×7=21 (which matches the next two digits '2' and '1')
5×6=30 (which matches the next two digits '3' and '0')
Following this exact rule:
The next pair is 8×1=8 (which should be represented as a single digit or matching value).
Thus, the next number/digit after 1 is 8.