Sentence Correction (SC) | Re: QOTD: Despite Japan’s relative isolation

In History, 'at that time' can refer to an era, and in this context an era that ran for nearly 300 years. It may be noted that the text talks about a prolonged peace and can it be logically expected that such an extended peace was obtained at a specific point of time?

Since the topic is from GMAT Prep, I hope you are not having any doubts about its veracity. Similarly, how we can take that D is superior when after all, D is also using "at that time", "during the shogunate"