A capacitance of 100 uF is connected in series with a resistance of 8000 ohm. The time constant of the circuit is

The correct answer is $\boxed{\text{A. }0.2\text{ s}}$.

The time constant of an RC circuit is the time it takes for the voltage across the capacitor to reach 63.2% of its final value. It is calculated by the formula $\tau = RC$, where $R$ is the resistance in ohms and $C$ is the capacitance in farads.

In this case, $R = 8000\Omega$ and $C = 100\mu\text{F} = 10^{-6}\text{ F}$. Substituting these values into the formula gives $\tau = 8000\Omega \times 10^{-6}\text{ F} = 0.2\text{ s}$.

The other options are incorrect because they do not correspond to the time constant of the circuit.