Throughout the course of the war, by steadfastly

Let's look well to analyse the options one after the other in order to pick the most suitable.

In Option A - INCORRECT. In this option, it is easily seen that there is no clear difference between the Option A and the question raised. More importantly, the comma sign immediately after 'German' fits appropriately as it is a separator but the addition of ''and'' to treat line of the statement makes it an overemphasized statement.

Since the first option is not correct, we look up to Option B - INCORRECT in a bid to getting a correct answer too. Unfortunately, just a mere addition of the preposition ''by'' does not give the extra information needed. Just an almost duplication of the question.

In Option C - CORRECT. This happens to appear the most concise because it explains without much technicality the parts and organs of the statement. The summary however appropriate also has a sizable punctuation effects.

From Option C we move to Option D - INCORRECT, the later part of the statement meant that the subject here Churchill was not the only major actor, that there exists some other principal actors too. This is inconsistent with the writer's position.

In the last solution, Option E - INCORRECT, We can say Churchill who's the subject here is being explained to have steered some other countries aside Britain which is also speculative and not consistent with the write-up.