The third side of the triangle can be determined using the triangle inequality theorem, which states that the sum of the lengths of any two sides of a triangle must be greater than the length of the third side.
In this case, the two given sides are 10 mm and 7 mm. Therefore, the sum of these two sides is 10 mm + 7 mm = 17 mm. The length of the third side must be less than 17 mm.
Among the choices given, the only option less than 17 mm is 5 mm. Therefore, the length of the third side is 5 mm.
So, the correct answer is:
5 mm