The third number in the sequence would be 7.
The sequence is gaining 2 from the first to second, 4 from the second to third, 6 from third to fourth and 8 from fourth to fifth.
1 + 2 = 3
3 + 4 = 7
7 + 6 = 13
13 + 8 = 21
21 + 10 = 31
31 + 12 = 43
This is the pattern in which the sequence follows and the answer is C. 7
