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

    Incorrect.
If students are paid $300 for writing a counter-attitudinal essay, they will probably not maintain attitude change.  Students paid $300 have sufficient justification for writing the essay, so they do not experience cognitive 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 (i.e., 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-attitudinal behavior (i.e., I believe what I wrote).