What is the complete prepositional?
The preposition, the object of the preposition, and everything
in between.
The object of the preposition answers the question
"(preposition) what?"
For example:
He looked in the box worriedly.
"in the box" is the prepositional phrase because "in" is the
preposition, and "box" is the object of the preposition.
"Box" answers the question, "(preposition) what?, or in this
case, "In what?"