Which observation would indicate that a burning candle has undergone a chemical change

If the candle turns into a liquid

Explanation:

A chemical change is a change of phase, and when a candle is burning/heated it melts  which is a chemical change.

a- all cells come from pre-existing cells

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products