Here is the item again:
Karlene drove slowly through the heavy traffic.
(A) Anticipating a bag
of microwave popcorn (B)
and a couple of hours in front of the television
(C) as her reward for
a hard day at work.
You correctly realized that the period after traffic created a fragment. Anticipating a bag of microwave popcorn ... is a participle phrase, so it cannot stand alone as a complete sentence.
To fix this fragment, you chose to do this:
Karlene drove slowly through the heavy traffic,
anticipating a bag of microwave popcorn and a couple of hours in
front of the television as her reward for a hard day at work.
You have correctly employed Punctuation Rule 3:
Main clause + , + participle phrase.
Keep up the beat!