This strengthens Tamara’s argument slightly by showing that more than just wealthy Japanese people appear to have feared ninjas. We instead need an answer choice that weakens her conclusion that most Japanese people feared ninjas in the Tokugawa period.

Even if their squeaky floors weren’t an effective defense against ninjas, wealthy Japanese people still seem to have feared them. The questions remains whether these wealthy Japanese accurately represented all Japanese people during the Tokugawa period.

This weakens the argument by showing that Tamara's assumption that wealthy Japanese accurately represent all Japanese people is false. (C) points out that Tamara can't draw a conclusion about all Japanese people in the Tokugawa period based only on evidence about wealthy people.

Whether or not ninjas were truly great fighters doesn’t change the fact that many wealthy Japanese people seem to have been afraid of them during the Tokugawa period. (D) doesn’t weaken Tamara’s conclusion or point out that her assumption is false.

Tamara’s conclusion is only about Japanese people during the Tokugawa period, so the presence or absence of ninjas at any other time period is not relevant.