This script prints the indices of the selected polygons of the active polygon object to the console.

get_list_of_selected_polygons.py

import c4d

def main():
    if op is None:
        print("No object selected.")
        return True
        
    if op.GetType() != c4d.Opolygon:
        print("Selected object is not a PolygonObject.")
        return True
    
    polySelection = op.GetPolygonS()
    if polySelection is None:
        print("Could not get polygon selection!")
        return True
    
    if polySelection.GetCount() == 0:
        print("No polygons are selected.")
        return True
        
    polyCount = op.GetPolygonCount()
    selection = polySelection.GetAll(polyCount)
    listOfSelectedPolygons = []
    for i in range(polyCount):
        if (selection[i] == 1):
            listOfSelectedPolygons.append(i)
        
    print(len(listOfSelectedPolygons), "Selected polygons:", listOfSelectedPolygons)
    
    return True


if __name__=='__main__':
    main()