How long until the current is at a maximum?

ANS:B - 5 s

Time constants in electrical circuits
In an RL circuit composed of a single resistor and inductor, the time constant Ï„ (in seconds) is

where R is the resistance (in ohms) and L is the inductance (in henries).
Similarly, in an RC circuit composed of a single resistor and capacitor, the time constant Ï„ (in seconds) is:

where R is the resistance (in ohms) and C is the capacitance (in farads).