Which set of three numbers could be the side lengths of a triangle?

a.2, 4, 8
b.3, 5, 7
c.2, 4, 6
d.3, 5, 9

B is the answer. The sum of two triangle legs should be more than the legth of the third.

For any triangle, the third side must be greater than difference of two sides and less than the sum.

Let the third side be x.

for 2, 4           x > 2,     x < 6.      
8 does not fit in, so 2,4, 8 can't form triangle

for 3, 5           x > 2,     x < 8.      
7 does fit in, so 3,5, 7 can form a triangle.

Testing the others in the same, doesn't form the sides of a triangle.

So only set B 3,5,7 could be the side lengths of a triangle.