The correct form of the verb to fill in the blank is “burning.”
So the complete sentence is: “As I entered the room, the first thing that struck me as odd was the candle still burning on the table.”
Here, “burning” is the present participle used as an adjective to describe the candle that was actively aflame.
This form correctly conveys the ongoing action at the moment of entering the room.
Using “burning” instead of other forms ensures the sentence is grammatically correct and paints a vivid picture of the scene.
Thus, the present participle “burning” appropriately fits the context of the sentence.