With regard to Class D control zones;
If the conditions are VMC then one can operate VFR or IFR.
If the conditions are IMC (less than VMC) but above your minima for special VFR you can operate special VFR if ATC have the capacity.
Otherwise you operate IFR.
Being in rain does not necessarily mean IMC - rain is not cloud and the vis could still be above your minima for VMC in class D.
If given a VFR clearance, one must remain VMC and if it appears that it will be unable to continue VMC, remain VMC if possible and request an IFR or Special VFR clearance.
Regards,
DFC