i) Constitutive enzymes are the ones produced by the cell under any conditions. For example, enzymes of glucose breakdown.
Induced enzymes are the ones produced by the cell only under certain circumstances. For example, enzymes for lactose breakdown are made by the cell only if lactose is present in the environment.
ii) Metabolic regulation is activation or inhibition of activity of enzymes based on the amount of their product or product of enzymes from the same pathway in the cell.
iii) Product inhibition is a type of metabolic regulation, when the final product of the metabolic pathway inhibits the first enzyme in the chain.