Step 1: Use the relationship between speed, distance, and time.
The speed of light in vacuum is \( v_{\text{vacuum}} = \frac{d}{t} \), and the speed of light in the denser medium is \( v_{\text{medium}} = \frac{5d}{T} \). The critical angle \( \theta_c \) is related to the refractive indices of the two media by the equation:
\[
\sin(\theta_c) = \frac{v_{\text{vacuum}}}{v_{\text{medium}}}
\]
Step 2: Substitute the values.
Substitute the expressions for the speeds into the equation:
\[
\sin(\theta_c) = \frac{\frac{d}{t}}{\frac{5d}{T}} = \frac{T}{5t}
\]
Thus, the critical angle is:
\[
\theta_c = \sin^{-1} \left( \frac{5t}{T} \right)
\]
Step 3: Conclusion.
Thus, the critical angle of the given pair of media is \( \sin^{-1} \left( \frac{5t}{T} \right) \), which corresponds to option (B).