A. The guards spotted the thief and was running away.
B. The guards spotted the thief running away.
C. The guards spotted the thief who was running away.
D. None of these
Explanation
The sentence "The guards spotted the thief who was running away" correctly connects the two ideas with the relative pronoun "who" to show that the thief was running away at the time of being spotted.
This maintains proper sentence structure and meaning.