Correct option is A
Given:

Logic: Add the numbers of the first and third figures of the same corners to get the number of the second figure of the same respective corner.
Explanation:
1 + 8 = 9
1 + 2 = 3
2 + 6 = 8
1 + 9 = 10
Similarly, 1 + 5 = 6
So, 6 is the missing number in the given figure.
Thus, the correct answer is (a).






