Demand of skateboard decreases it means, most of the people in that location have skateboard or it is replaced by that item which is more worthy and having reasonable price and more efficient than that skateboard.
Being a citizen of earth he or she will keep in mind that it uses minimum amount of plastic and if that product is made up of plastic it is biodegradable plastic.
So, if this is the case , then price of the skateboard will decrease.
Option D: Decrease