If you need an IFR transit because you are in IMC and flying IFR you must ask for it. Otherwise just ask for zone transit and they'll ask if VFR or IFR, VFR is easier for them because the separation criteria are much lower and leave you with see and avoid responsibility for other VFR and IFR traffic.
Class D rules are
•IFR and VFR flights are permitted
•All flights are provided with air traffic control service
•IFR flights are separated from other IFR flights and receive traffic information in respect of VFR flights
•VFR flights receive traffic information in respect of all other flights.