The correct answer is (C) 12
Explanation:
As it is an isosceles triangle and you can see it is divided into two right angled triangle, therefore, we can use Pythagoras theorem here:
[tex]10^2 = 8^2 + ( \frac{x}{2})^2
\\
100 = 64 + x^2/4
\\
x = \sqrt{144} = 12
[/tex]
NOTE: (x/2) because the triangle base is divided into two halves.