18 cm is correct .
Step-by-step explanation:
The sum of two smaller sides is greater that the largest side.
It is given that the two sides of a triangle measure 8 cm and 15 cm.
Case 1: Let 8 cm and 15 cm. are smaller side. So,
Third side < 8 + 15
Third side < 23
It means 3rd side must be less than 23
Case 2: Let 15 cm is the largest side.
15 < Third side + 8
15 - 8 < Third side
7 < Third side
It means 3rd side must be greater than 7.
Since only 18 is less than 23 and greater than 7, therefore the possible length of third sides is 18 cm and option 2 is correct.