Final answer:
The current in an RL circuit is proportional to e^{-t}, growing exponentially as it approaches a steady state value when a voltage source is present.
Step-by-step explanation:
The relationship between current (I) and time (t) in an RL (resistor-inductor) circuit when a voltage source is present is characterized by an exponential function. The time variation of the current, after a switch is closed and the circuit is first powered on, can be described by the equation I(t) = Io(1 - e^{-Rt/L}), where I is the current at time t, Io is the maximum current (V/R), R is the resistance, L is the inductance, and e is the base of the natural logarithm. Therefore, the correct answer to the student's question is I∝e^{-t}, indicating the current grows and eventually approaches a steady state over time following an exponential increase.