The verb form needed is “burning.”
The full sentence is: “As I entered the room, the first thing that struck me as odd was the candle still burning on the table.”
“Burning” acts as an adjective, describing the candle's active state.
This form accurately reflects the ongoing action when entering the room.
Using “burning” ensures grammatical correctness and creates a clear image.
Therefore, the present participle “burning” suits the sentence's context.