Final answer:
The best supporting quote is "d. Mr. Nelson's conducting had a calming effect on Brennan," as it implies that watching the conducting helps manage his tics.
Step-by-step explanation:
The quote that best supports the idea that Brennan watches Mr. Nelson's conducting in an effort to hold back his tics is "d. Mr. Nelson's conducting had a calming effect on Brennan."
This choice directly suggests that the act of watching Mr. Nelson's conducting helps Brennan manage his involuntary noises and movements, which is indicative of it having a soothing or stabilizing influence on him. Options a, b, and c do not directly address the idea of suppressing tics, although they relate to how Brennan engages with Mr. Nelson's conducting.