We have a right triangle with legs of 6 and 8. The hypotenuse is unknown so let it be x for now
Plug these three values
a = 6
b = 8
c = x
into the Pythagorean Theorem below. Then solve for x
a^2 + b^2 = c^2
6^2 + 8^2 = x^2
36+64 = x^2
100 = x^2
x^2 = 100
sqrt(x^2) = sqrt(100) ... apply the square root to both sides
x = 10
The final answer is 10 ft. This option isn't listed which makes me think your teacher made a typo somewhere.