Review the passage:
While cleaning under his bed, Glen heard the vacuum cleaner
suck up something hard and metallic. Glen wanted to know what the
object was. But had no intention of digging through a bag of dust, dead
bugs, cat litter, and bathroom hair to find out.
You chose the first part, While cleaning under his bed, Glen heard the vacuum cleaner suck up something hard and metallic. This part has a main clause: Glen is the subject; heard is the verb. A fragment will not have a main clause.