Which set of numbers cannot represent the lengths of the sides of a triangle A) 7,9,12 B) 8,6,7 C) 8,20,12 D) 10,13,20

Hi Friend

You should make use of the Triangle Inequality Theorem, which states that

the sum of the lengths of two sides of a triangle must always be greater than the length of the third side

As you test the answers, you will notice that in C 7+11=18, which is also equal to the length of the other side. This is impossible so the answer is C.