[amp_mcq option1=”0.2 s” option2=”0.4 s” option3=”0.6 s” option4=”0.8 s” correct=”option3″]
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.