Resonance occurs in a series LCR circuit when the frequency of the applied emf is 1000 Hz.

$(\mathrm{f}=900\mathrm{Hz})<\left({\mathrm{f}}_{\text{resonance }}=1000\mathrm{Hz}\right), \Rightarrow {\mathrm{X}}_{\mathrm{C}}>{\mathrm{X}}_{\mathrm{L}}$ and the circuit becomes capacitive and current leads emf at frequency of 900 Hz.
At resonance, impedance is minimum (Z=R).
Not only at resonance, but at all conditions voltages across L and C differ in phase by 180^o when connected in series.

$f_{\text{resonance}}\propto \frac{1}{\sqrt{C}}$. Therefore when C is doubled, the resonance frequency will decrease, i.e., will be at a frequency less than 1000 Hz.