After a voltage has been applied to an inductor, the current will be ___ of its full value after 2 time constants.

ANS:B - 86%

Since in an inductor the current increases as it reaches transient the formula is, IL=IT(1-e^(-t/T)).
t = total time.
T = time constant = L/R.
Since the problem is only concerned about the percentage, you can just use (1-e^-(t/T)).
(1-e^(-2T/T)) = 1-e^(-2) = 0.86466 = 86% thus the answer, option B.