Take a look at the item again:
While Dr. Kinser bounced down the stairs
(A) and strode through the
hallway. (B)
A brightly colored tie streamed over his
shoulder, (C) flapping
in his wake.
You wanted to do this:
While Dr. Kinser bounced down the stairs and strode through the hallway.
A brightly colored tie streamed over his shoulder
flapping in his wake.
Removing the comma after shoulder is an error. Flapping in his wake, a participle phrase, needs the comma to connect it to the main clause in front.
You might want to review the rules.
Go back to the sentence to try again.