Candle is a solid cylindrical block of wax with a wick embedded in it.
a) A candle starts melting when it comes in contact with the heat of the flame. The change in shape and size of the candle takes place. This indicates a physical change from solid state to liquid state. It is possible to get the wax back after it melts.
b) A wax candle acts as a fuel when we burn the candle. It is basically carbon. The wax oxidises (burns) in the flame to yield water and carbon dioxide, which dissipate in the air around the candle in a reaction, which also yields light and heat. The unburnt carbon is deposited as a black substance called soot. This signifies a chemical change. It is not possible to get the wax back after it burns.