B: Students are paid $300 for writing an essay that goes against their
    beliefs.

    Incorrect.
If students are paid $300 for writing a counter-attitudenal essay, they will probably not maintain attitude change.  Students paid $300 have sufficient justification for writing the essay, so there is no dissonance.    Recall the classic study by Festinger and Carlsmith (1959) (see Myers pg. 153), where subjects were given dull tasks to perform for an hour and then given either $1 (which is now about $5) or $20 (which is now about $100) to lie to the next subjects (who were waiting to participate in the experiment) and say the experiment was interesting.  When participants in the $20 condition were asked how much they liked the experiment, they liked it far less than the subjects who were given $1.  The reason the participants given $20 did not like the experiment very much is that they had sufficient justification ($20) to lie, and say the experiment was interesting, while those paid $1 to lie were given insufficient external justification to lie (only $1), thus they created an internal justification for their lie (e.g. I liked the experiment).  In question 9, the students who were paid $300, have sufficient justification to write an essay that goes against their beliefs.  If the students were given $1, or a candy bar, or something else very small, they would probably maintain attitude change, because they would search for an internal reason for their counter-attitudenal behavior (e.g. I believe what I wrote).