The correct y-coordinate of point $B$ is $\frac{\sqrt{3}}{2}$, which corresponds to option C.

Would you like a detailed explanation or have any questions?

Here are 5 related questions for deeper understanding:

1. What is the significance of the unit circle in trigonometry?
2. How do you convert radians to degrees, and why is this useful in solving geometry problems?
3. How do sine and cosine functions relate to points on the unit circle?
4. Can you derive the x-coordinate of point $B$ using the given information?
5. What are some common angles in radians, and how are they represented on the unit circle?

Tip: Remember, when dealing with unit circles, the coordinates of a point can be found using trigonometric functions of the angle.