The reverse saturation current in a diode can be calculated using Ohm's Law:
\[
I = \frac{V}{R}
\]
where \( I \) is the current, \( V \) is the applied reverse bias voltage, and \( R \) is the resistance of the diode.
Given:
\[
V = 8 \, \text{V}, \quad R = 4 \times 10^7 \, \Omega
\]
Substitute the values into the equation:
\[
I = \frac{8}{4 \times 10^7} = 0.2 \times 10^{-6} \, \text{A} = 0.2 \, \mu A
\]
Thus, the reverse saturation current is \( 0.2 \, \mu A \).
Therefore, the correct answer is \( \boxed{0.2 \, \mu A} \).