Which of the following side lengths can form a triangle?

16 cm, 21 cm, and 47 cm

1 in, 2 in, and 3 in

34 mm, 75 mm, and 100 mm

8 ft, 24 ft, and 33 ft

34 mm, 75 mm, and 100mm side lengths can form a triangle.

This is because of the "triangle inequality rule", which states that the sum of two side lengths in a triangle must be greater than (not less than or equal to) than the third side length.
A + B > C 
A + C > B 
B + C > A

I'm gonna have to say about 13 percent