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, which 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!