Thank about it this way, you have 3 logs, each different lengths, and you lay them next to each other. The longest is on its own and the two smaller ones stacked on top of each other beside it. In order to make a triangle, there must be at least a slight angle coming off the longer log - this means the sum of the other two must be larger than the longest one, otherwise a triangle cannot be made.
A) 5 - 2, 3
2 + 3 = 5
5 = 5 therefore no
B) 9 - 7, 3
7 + 3 = 10
9 < 10 therefore yes
C) 9 - 5, 4
5 + 4 = 9
9 = 9 therefore no
D) 15 - 7, 6
7 + 6 = 13
15 > 13 therefore no
B is the set of numbers
