How to pass an object between forms with winforms using c#?

I am trying to pass object from one from to another.

so here is what I have done.

from form 1

            if (i._itemType == "Phone Activation") {

                using (TrackInfoPhoneActivation form2 = new TrackInfoPhoneActivation()) {
                    if (form2.ShowDialog() == DialogResult.OK) {
                        AdditionalInfoPhoneActivations obj = form2.AdditionalInfoPhoneActivationsObject();
                    }
                }

            }

and from on the TrackInfoPhoneActivation form, I have this

public AdditionalInfoPhoneActivations AdditionalInfoPhoneActivationsObject() {
   var t = new AdditionalInfoPhoneActivations();
   t.phoneNumber = InputPhoneNumber.Text;
   t.serialNumber = InputSerialNumber.Text;
   t.saleType = InputSaleType.SelectedValue.ToString();
   t.carrierID = (int)InputCarrier.SelectedValue;
   return t;

}

but I keep getting this error

Error   1   Inconsistent accessibility: return type 'RM.AdditionalInfoPhoneActivations' is less accessible than method 'RM.TrackInfoPhoneActivation.AdditionalInfoPhoneActivationsObject()' C:\Users\User\C# Projects\RM\RM\TrackInfoPhoneActivation.cs 23  47  RM

What am I doing wrong here? and how can I correctly pass this object from AdditionalInfoPhoneActivations to form1?