Answer:
6, 12, 15 could be used to form a triangle
Explanation:
The side measurements could be used to form a triangle if they form a triplet i.e they obey the inequality,
a + b > c
I.e. sum of any two sides is greater than the 3rd side
Trying the sets, we have,
1,5,9
1 + 5 < 9
So, this can't be a triangle
3, 17, 21
3 + 17 = 20 < 21
this can't be a triangle
6, 12, 15
6 + 12 > 15
12 + 15 > 6
15+ 6 > 12
This could be used to form a triangle
9, 8, 17
9 + 8 = 17 since this is not greater than 17,
this can't be a triangle