Answer: (D) She should verbally thank all of the workers she met for their time.
Step-by-step explanation:
This is simple. We can just cross out the ones we know are wrong and think, "Would she tell her friends about the shadowing experience to thank the workers, possibly making them use up more of their time? Nah" So then the only possible answer would be D.
(TL;DR) The answer is D.