Final answer:
The phrase that contributes to the informal tone is C. 'furiously scribbling notes'.
Step-by-step explanation:
In formal writing or speech, it is more common to use words like 'quickly' or 'hastily' instead of 'furiously'. The word 'furiously' conveys a sense of urgency or intensity, which adds informality to the tone of the sentence.
For example, in a more formal tone, the phrase could be written as 'quickly taking notes' or 'hastily writing down notes'.
The use of 'furiously' in this context creates a casual, conversational tone that is often associated with informal writing or speech. Therefore the correct answer is C. 'furiously scribbling notes'.