In the context of alternating voltages and currents, which device typically uses direct current (DC) to operate?

Which of the following devices is most likely to operate on alternating current ($\mathrm{AC}$)?

An AC source produces an alternating current in a circuit with the function $i\left(t\right)=\left(1.5A\right)\cos\left\lbrack\left(250s^{-1}\right)t\right\rbrack$. What is the frequency of the source? What is the maximum current in the circuit?