The answer is C: The young man is annoyed by the children and the activity in which their aunt engages them.
In the first paragraph of Saki's short story, the bachelor sits as far away from the other occupants as he can. They are described in this manner:
Both the aunt and the children were conversational in a limited, persistent way, reminding one of the attentions of a housefly that refuses to be discouraged.
Obviously, they are annoying as they talk. Then the boy does not sit quietly, but instead he pounds a cushion making the dust rise about the compartment of the train; his aunt keeps saying "Don't" to the children who respond with "Why?" indicating that they fear nothing from her and are not respectful.
It is clear that this bachelor is going to have a frustrating ride as he is already annoyed by the lack of discipline which the aunt exerts over the unmannerly children who crowd the compartment and are impolite in their chatter as well as disrespectful to their inept aunt.
What follows in Saki's clever story within a story is quite humorous as it demonstrates the power of understatement.