The sentence that has an error in comparison pronoun case is:
D Janet, always jealous of Barbara's popularity, thinks of herself as actually being much prettier than Barbara.
In this sentence, the use of "herself" is correct as it refers back to "Janet." However, the phrase "much prettier than Barbara" creates a comparison that could be misleading regarding pronoun case, especially if one considers other possible structures. Nevertheless, if we were to pinpoint a clearer error based on strict grammatical rules, none of the sentences outright misuse pronoun cases in a traditional sense, but they do exhibit nuances in clarity.
Thus, upon closer examination, the sentences largely maintain appropriate pronoun cases—meaning none outright contain a glaring error typical of comparison pronoun case misusage. Each sentence's structure holds its own logic and consistency. If comparisons are strictly being examined for grammatical case rather than clarity, none particularly stand out.
That said, in terms of clarity, the ambiguity in D might suggest a need for rephrasing to avoid confusion, as "herself" is used a bit awkwardly compared to "she" in a traditional comparative sentence setup.