A

To find the critical angle \( \theta_c \) for a material with refractive index \( n \), we use Snell's Law, which states: \( n_1 \sin(\theta_1) = n_2 \sin(\theta_2) \). At the critical angle, \( \theta_2 \) is 90 degrees, making \( \sin(\theta_2) = 1 \). Therefore, the equation simplifies to:
\[ n \sin(\theta_c) = 1 \]
From here, we can express the critical angle as:
\[ \sin(\theta_c) = \frac{1}{n} \]
Hence, the critical angle can be calculated using:
\[ \theta_c = \arcsin\left(\frac{1}{n}\right) \]
Therefore, the answer is option A.