A) If you only sample people in a sports supply store, your sample will be biased. People in a sports supply store are more likely to be people that belong to a gym.
B) While it does not have necessarily the same amount of bias as sampling people in the sports supply store, people that go to a park are generally more likely to be people that exercise or have a gym membership.
C) Taking a random sample of people in town is a good way to get a non-biased sample. They are not necessarily predisposed to answer your question one way or another.
D) For the same reason as choice A, this is a biased sample.
C is the best choice.

divide the numerator and denominator by 13
=
SA= Lateral Area + Area (of base)
Lateral area = perimeter of base x height
An isosceles triangle has two congruent base segments. The hypotenuse of an isosceles right triangle is (length of base segment) x (the square root of 2). I did the math for you (shown above) to get 5 as a base.
So 5 + 5 + square root of 50 is your perimeter
Multiply the perimeter times your height (8) and you get the lateral area.
Add your lateral area to the base area (triangle = 1/2 bh)
LA = 136.568...
Area of Triangle base = 12.5
Surface area = 149.068... square cm