A

Step 1: Total Internal Reflection occurs when light travels from a medium of higher refractive index to one of lower refractive index.
Step 2: Using Snell's Law, \( n_1 \sin \theta_1 = n_2 \sin \theta_2 \). Here, \( n_1 = 1.25 \) (for medium 1) and \( n_2 = 1.0 \) (for air).
Step 3: At the critical angle, \( \theta_2 = 90^{\circ} \), so \( \sin \theta_2 = 1 \). Substituting in Snell's law gives us:
\[ 1.25 \sin \theta_c = 1 \]
Step 4: Solve for \( \sin \theta_c \):
\[ \sin \theta_c = \frac{1}{1.25} = 0.8 \]
Therefore, the answer is \( \sin \theta = 0.8 \).
Hence, A.