Irrelevant. The argument is about how certain writings are treated when they could hypothetically be widely influential. Being believed or being popular is different from being influential. Even if (A) were on topic, there’s no need to assume what’s likely sufficient or necessary for being influential in order to reach a conclusion about how hypothetically influential writings are treated.

If anything, (B) runs slightly counter to the argument. The historian must assume that at least some regimes that censor on moral grounds are, in fact, totalitarian regimes. Otherwise, his premise about regimes that censor would tell us nothing about totalitarian regimes.

The support is that regimes will forbid things they perceive as a threat to their power, while the conclusion is that totalitarian regimes will forbid things that reduce public passivity. So the historian must assume that if a thing reduces public passivity, totalitarian regimes at least sometimes perceive it as a threat to its power.

The argument is about how regimes respond to perceived threats to their power. Actual requirements for power and threats to that power are irrelevant. What matters is whether a regime percieves public passivity as important to its power.

This independently supports the conclusion, but it’s not an assumption. The concepts here—writings, things labeled blasphemous/pornographic, and things that reduce public passivity—only appear in the conclusion. (E) has nothing to do with the relationship between support and conclusion, so it isn’t an assumption.