Concept:"Run into" means to meet or encounter someone unexpectedly.
This idiom is the correct key to completing the sentence.
Explanation:The thief was running away, so the missing word must show what happened when he met the police.
"Ran upon," "ran by," and "ran through" describe movement near or across the police, not an accidental meeting.
"Ran into the police" clearly means that he suddenly met or came across them while escaping.
Thus, "into" fits the meaning of the sentence perfectly.
Answer:C. into