1.  Which one will replace the question mark ?

A. 18
B. 20
C. 21
D. 19

Answer: Option B

Explanation:

1 + 2 + 3 + 4 = 10

and 1 + 3 + 5 + 8 = 17

Similarly, 1 + 4 + 6 + 9 = 20.