What happens to the demand for products whenever the price of
a
substitute item decreases?

Demand decreases

Explanation:

Substitute goods are products that can be used in place of each other. Goods are described as substitutes if a customer can use them interchangeably and get equal or almost the same satisfaction. Tea and coffee will be substitutes if a customer can consume either of them and be happy.

If the price of a substitute good declines, customers will prefer consuming it instead of the other product. The other product's demand will decrease due to a change in customer preferences as a result of a lower price.

it's obviously d. it could be c though.