Irrelevant. We're not interested in the broadsides' literary quality. We're interested in whether the broadsides tell us anything about the moral attitudes of their buyers, a question this answer choice doesn't help us answer.

This strengthens the argument. This answer choice doesn't tell us for sure that people purchased these ballads only for entertainment value, and besides, we don't know how many of this kind of ballad there were. But this answer choice still provides useful context for the fact that "over half of surviving broadsides contain moralizing statements." If a significant proportion of those broadsides only contained one, not very convincing, moralizing statement, this further undermines any case that the high proportion of broadsides with moralizing statements proves something about seventeenth-century moral attitudes. Thus, (B) strengthens the author's argument.

Irrelevant. We don't know how common this practice was, or how popular these sermons were compared to broadsides.

Irrelevant. We're not interested in the use the clergy "occasionally" made of these broadsides — we're interested in moral attitudes of the population more generally.

While this might be tempting, keep in mind that we don't know how many "well-educated" people there were in the seventeenth century. They might have been a very small portion of the population, so their attitudes don't necessarily show anything about the moral attitudes of "most seventeenth-century people." We also don't know why these well-educated people looked down on broadsides. They might not have had moral reasons so much as aesthetic or class-related reasons. So this answer choice doesn't effectively strengthen the argument.