Final answer:
Miss Bingley's conversation blunders include making snide remarks about Elizabeth's family and boring Darcy with too much talk on irrelevant topics, ultimately failing to sway Darcy against Elizabeth and showing her own shortcomings. The correct option is A.
Step-by-step explanation:
The two mistakes Miss Bingley makes in her conversation that backfire in her attempt to discredit Elizabeth before Darcy are A) making snide remarks about Elizabeth's family and social standing and B) talking too much and boring Darcy with irrelevant topics. Miss Bingley intends to diminish Elizabeth in Darcy's eyes, but her strategy is flawed.
By speaking ill of Elizabeth's relatives, she not only fails to turn Darcy against Elizabeth but also reveals her own unkindness.
Additionally, Miss Bingley's excessive talking about matters that do not interest Darcy only serves to highlight her lack of discernment and inability to engage with Darcy on a more intellectual level. These blunders ultimately contribute to her unsuccessful bid for Darcy's affections.