Answer
Choice A.
Work Step by Step
As seen in the previous problem, the current in the circuit is $i=i_0e^{-Rt/L}$.
Solve for the time $t_{0.5}$ for the current to reach one-half of its original value.
$$i=i_0e^{-Rt/L}$$
$$0.5i_0=i_0e^{-R t_{0.5}/L}$$
$$ t_{0.5}=(L/R)ln2$$
In the situation described here, the resistance R is increasing as the magnet loses more and more of its superconducting ability, so the time $t_{0.5}$ will be shorter.
