which of the following sets of line segments could make a triangle?

i don't get it could you explain please
a. 3,5,4
b. 1,3,2
c. 12,20,7
d.14,11,15

a) 3, 4 , 5 is a common right triangle

b) will not quite work
3 is long side
1 + 2 just barely stretches from end to end, just making a thicker three inch line :)

c) misses by an inch. 7 + 12 = 19 which will not reach from end to end of 20

d) sure, why not - draw it