Capacitor Maximum Frequency of Operation
You followed all the instructions and got the correct answer that they requested.

But the question has false assumptions.

The cap is very good and is low Q ~1. Thus there is no significant resonance at the crossover frequency when the magnitude of impedance for L=C.  This means the impedance 1 decade above is about the same as 1 decade down is still useful .



Of course if you are using this cap. with a Buck Converter then the effects of the series inductor and interference with the loop filter spectrum  must be simulated to avoid stability issue .



Here SRF of 25kHz with a Q of 22 dB into a 10 Ohm load @  for a Buck SMPS perhaps operating much higher than  25kHz yet has good attenuation higher above this.