Answer:
a. a rightward shift of the demand curve for margarine
Explanation:
If the price of butter increases, consumers would demand less of butter and more of margarine. This would shift the demand curve of margarine to the right and the demand curve of butter to the left.
A substitute good is a good which can be used in place of another good. Substitute goods usually have a more elastic demand because if the price of the good increases, it can be easily substituted with another good.
The phenomenon exhibited by butter and margarine is known as cross price elasticity. It when the change in price of one good leads to a change in the quantity demanded of another good.
