Correct option is C
Given - 10, 20, 10, 9, 21, 11, 8, 22, 12, (...)
In the given series there are three series-
1st series – 10, 9, 8, , ……………
2nd series – 20, 21, 22, , ………...
3rd series – 10, 11, 12, …………
Therefore, the correct option is (c).
missing term will be in first series - that is 7