At what temperature does the solid start melting? –20°c 0°c 20°c 80°c

Therefore, At 0 °C solid starts melting.

Explanation:

The melting point of a substance is the temperature at which it changes state from solid to liquid.

Different substance have different melting point.

The melting point of ice at 1 atmosphere of pressure is very  to 0 °C  this is also known as the ice point.

alchemy hadn't scientific methods of study, appeal to mysticism and fantasy, the knowledge of chemistry and physics were modest at that time.

alchemy was - but as new rules were discovered it either changed into chemistry or it became "unscientific".