1.) The United States suffered the greatest losses on D-day
2.) I think the D Day landings were made on beaches instead of harbors because the Allies could make a better surprise attack.
3.) The biggest obstacles the Allies had to overcome on D-day were naval mines that threatened boats trying to land, minefields and German sldiers waiting on the cliffs.
Hope this helps!
mssg me for any additional info or anything that i left out. :P