The correct answer is: C. Rises to 63.2% of its final steady value.
The time constant of a capacitance circuit is the time it takes for the voltage across the capacitor to reach 63.2% of its final steady-state value. This is because the capacitor charges exponentially, and the time constant is the time it takes for the exponential function to reach 63.2% of its final value.
Option A is incorrect because the voltage across the capacitor never falls to 36.8% of its final steady value. The voltage across the capacitor starts at 0% and then increases exponentially to its final steady-state value.
Option B is incorrect because the voltage across the capacitor never rises to 38.6% of its final steady value. The voltage across the capacitor starts at 0% and then increases exponentially to its final steady-state value.
Option D is incorrect because the time constant is defined as the time it takes for the voltage across the capacitor to reach 63.2% of its final steady-state value.