Which set of numbers may represent the lengths of the sides of a triangle?

If you add up the two smaller sides, you should get a number greater than the longest side. Then you know it is a triangle.

For example: 3, 4, 5.
3 + 4 = 12.
12 > 5; so three, four and five make up a right triangle.

2, 3, 5 
2 + 3 = 5
5 is not greater than five, so that would be not be a triangle.