The sentence "Where was Mom anyway, and wasn’t there something she had to do?" shows irony because Matilda is oblivious to the fact that the situation she created by neglecting her chores has led to a chaotic scene. She should be focused on the potential danger and her responsibilities—like the oven that was left on and now potentially causing a fire—rather than questioning her mother’s whereabouts. This highlights her lack of awareness about the seriousness of her actions, despite knowing she had a duty to turn off the oven. Ironically, instead of fulfilling her responsibility, she is caught up in her laziness while the consequences are unfolding around her.
Thus, the most fitting response is:
The reader already knows that Matilda forgot to turn the oven off.