Final answer:
Upon doubling the current in an inductor, the new energy stored is four times the original energy; hence, the new energy stored is 4U.
Step-by-step explanation:
When an inductor with inductance L carries a current I, the energy U stored in the magnetic field of the inductor is given by the formula U = 1/2 LI². If the current flowing through the inductor doubles, the new current will be 2I. Using the original equation for energy stored in an inductor, the new energy stored U' would be calculated as U' = 1/2 L(2I)². This simplifies to U' = 1/2 L(4I²), which is 4 times the original energy U. Therefore, the new energy stored in the inductor when the current doubles is 4U, so the correct answer to the question is (c) 4U.