The correct option is C Boring and mundane matters
The 'people' mentioned in para 2 line 6 refers to Austen's warmest fans who, however, did not find her letters to their liking: personal letters which discussed matters a personal letter should. They found Austen's letters to her sister to be inadequate. These 'people' would find the examples of subjects that a personal letter should contain in para 2 lines 10-12 as boring and mundane as these examples are quite ordinary and discuss everyday, normal issues. Thus, in the given context, E is the best answer. The other options are incorrect as they do not fit the context of the passage.