D is incorrect because “The Battle of Britain” wasn’t a full scale attack, after invading France, Germany focused on bombing raids to soften Frances main ally (Britain). The bombing runs however were countered due to Britain’s invention-RDF Range and direction finding.
That leaves two major battles that both had significant turning points.
Operation Overlord otherwise known as D-Day was an offensive assault on the northern coastline of France. The offensive attack led to a victory; liberating France.
The Battle of the Bulge was also a turning point because it was Germany’s last major offense. The offensive attack was intended to stop Allied use of the Belgian port of Antwerp and to split the Allied lines, allowing the Germans to encircle the allied forces. Had the Germans thought it through, they may have launched a successful offense but due to the attack failing, Germany soon surrendered from the war.
Personally I’d choose C. Operation Overlord but then again A. The Battle of the Bulge May be correct, it depends how you interpret the question. I hope this helps and sorry for a history lesson (tried to sum it up).