Why A resonant (tuned) circuit is formed by an inductor and a capacitor together, either in series or in parallel. Energy sloshes back and forth between the magnetic field of the coil and the electric field of the capacitor, and at one particular frequency the inductive and capacitive reactances are equal and cancel. That resonant frequency is f = 1 / (2 x pi x square root of LC), which is why LC combinations are used to set frequency in filters, oscillators and antenna tuners.
Watch out A linear voltage regulator only holds a DC output steady, and a resistor network can absorb power but cannot store and exchange energy the way L and C do, so neither is frequency selective.
Tuned = L + C. Resistors dissipate, but only inductance and capacitance resonate.