The best option for Siobhan to handle the issue with her D string is:
B. She should tighten or loosen the pegs until the D string sounds right.
Before considering replacing the string or avoiding it entirely, it's important for her to make sure the string is properly tuned. If she’s unsure how to tune a guitar, she can use a tuner or a tuning app for assistance. Options A, C, and D are not advisable since they could prevent her from addressing the tuning issue effectively.