Using what is known as the "Triangle Inequality Theorem" a+b can not be less than c or else it is not a triangle. (A and B are the smallest sides of the triangle).
A) 6cm + 15cm = 21cm which is less than 23cm: Not a triangle
B) 5cm + 9cm = 14cm which is greater than 13cm: This makes a triangle and therefore the answer is B
C) 5cm + 8cm = 13cm which is less than 13cm: Not a triangle
