The correct answer is D.
German pilots used river Thames to help them orientate themselves in the air, and once they located the river they could follow it until reaching London and, even once in London, they could continue using the river as a reference, they could know which part of the city they were exactly bombing and even choose to bomb strategic locations such as industries, weapon-production facilities or means of transports.
The other options three options mentioned do not benefit the Germans, as they describe manners in which river Thames protected the British and not the other way around.