Your answer is A.
When you have a 30-60-90 triangle, your sides will always have a set pattern.
The side across from the 30 angle will be equal to n (in this case n = 4).
The side across from the 60 angle will always be equal to n√(3) (in this case, 4√(3)).
The side across from the 90 angle will always be 2n (in this case, 8).
You could go into trig or pythagorean theorem to prove this, but it's much simpler to remember it because it's a special triangle.