The painting you described, featuring a candle representing the eternal flame, primarily makes use of symbolism. Symbolism involves using objects, colors, or images to convey deeper meanings and concepts. In this case, the candle symbolizes the idea of an eternal flame, representing life, hope, spirituality, or memory, depending on the context.
While aspects of naturalism might be present in the way the candle is depicted, the central theme focuses on the symbolic interpretation of the candle rather than a realistic representation. Abstraction can also be a consideration, but in the context of representing the eternal flame, symbolism is the most relevant approach.