In order to find the length of the third side, we can use the Pythagoras theorem.
The Pythagoras theorem can be used in right triangles, and it is a relation between the hypotenuse (h) and the two legs of the triangle (a and b):
In the triangle given, the hypotenuse is 8 cm, and the legs are 5√2 cm and x.
So using the Pythagoras theorem, we have that:
So the value of x is √14 cm, therefore the correct option is C.