The question asks for the basic acute angle of $210^\circ$.

To find the reference (basic acute) angle for a given angle in standard position:

1. $210^\circ$ lies in the third quadrant.
2. The reference angle is calculated by subtracting the given angle from $180^\circ$.

$\text{Reference angle} = 210^\circ - 180^\circ = 30^\circ$

Thus, the basic acute angle of $210^\circ$ is $30^\circ$, which corresponds to option C.

Would you like more details or have any questions?

Here are 5 related questions to expand your understanding:

1. What are reference angles and how are they used in trigonometry?
2. How do you determine which quadrant an angle lies in?
3. Can angles greater than 360° also have reference angles?
4. How do trigonometric functions behave differently in different quadrants?
5. What is the difference between an acute angle and a reference angle?

Tip: Reference angles are always positive and between $0^\circ$ and $90^\circ$, regardless of the original angle's quadrant.