Correct Answer: A
Explanation:
The given sequence is a combination of two series :
I. 1, 3, 6, 10, (....)
II. 1, 9, 36, 100, 225
The patternin I is + 2, + 3, + 4,...
The numbers in II are squares of the corresponding numbers of I.
.'. Missing number = 10 + 5 = 15.