Question

according to adam smith, what happens when the demand for a product decreases

Answer 1

they will either stop the production of that product or start to sell it for a less price.

Answer 2

the supply goes down/the price decreases.
logically, it makes sense. if nobody wants french fries, they're going to stop making them or lower the price. think about it in terms of french fries. :)
high demand, low supply= high price
low demand, low supply= normal price
low demand, high supply= cheap price