Answer:
A is the correct set of coins
We can use the pythagorean theorem to solve this
The pythagorean theorem = a^2 + b^2 = c^2, while
a = one leg
b = the second leg
c = the hypotenuse
In this problem,
a = 3
b = 15
c = ?
Let's plug our values into the pythagorean theorem
(3)^2 + (15)^2 = c^2
9 + 225 = c^2
Add together the left side
234 = c^2
Take the sqrt of both sides
sqrt(234) = c
The length of BC = sqrt(234)
Notice the picture
we have, the opposite side
the angle
and we want the hypotenuse
so recall your SOH CAH TOA

which one has all that? low and behold, is Ms Sine,
so let's bother Ms Sine

make sure your calculator is in Degree mode, since the angle here is in degrees, as opposed to Radian mode